Tempo and meter belong together. Without both, there is no way to know where a beat lies in time.

Tempo provides a musical pulse, which is divided into beats and bars by a meter. When tempo is changed or an audio-locked meter is moved, all objects on the timeline that are glued to bars and beats (locations, regions) will move in sympathy.

When performing meter or tempo operations, it is advisable to use the BBT ruler (available by right-clicking an existing marker or ruler name), and ensure that the constraint modifier is set (in Preferences->User Interaction) so that no other modifiers share its key combination. The constraint modifier is the "Constrain drags using: " setting under the "When Beginning a Drag" heading. One viable setting is .

Tempo

Tempo can be adjusted in several ways:

by double clicking on a tempo marker. This opens the tempo dialog which allows entering the tempo directly into an entry box.
by using the constraint modifier (which is set in Preferences->User Interaction) to drag the beat/bars in the BBT ruler or the tempo/meter lines. This is the preferred way to match the tempo to previously recorded material.

When dragging the BBT ruler, musical snap has no effect, however be warned that non-musical snap is in effect if enabled. Snapping to a minute while dragging a beat may result in some verly slow tempos. Snapping a beat to a video frame however is an incredibly useful way to ensure a soundtrack is punchy and synchronised to the sample.

by holding down the constaint modifier while dragging a tempo vertically. This is used for more complex tempo solving, as it allows changing of the position and tempo of a tempo marker in the same drag; it is, however, a useful way to adjust the first tempo for a quick result.

A tempo may be locked to audio or musical time. This can be changed by right-clicking on a tempo. If a tempo is locked to music, an entry will be available to lock it to audio. Similarly an audio-locked tempo may be locked to music by right-clicking it and selecting the "Lock to Music" entry.

Audio locked tempo marks stay in their frame position as their neigbour's positions are altered. Their pulse (musical) position will change as their neighbours move. Music locked tempo marks move their frame position as their neighbours are moved, but keep their pulse position (they will move as the music is moved).

A tempo may be constant or ramped:

A constant tempo will keep the session tempo constant until the next tempo section, at which time it will jump instantly to the next tempo. These are mostly useful abrupt changes, and is the way in which traditional DAWs deal with tempo changes (abrupt jumps in tempo).
A ramped tempo increases its tempo over time so that when the next tempo section has arrived, the session tempo is the same as the second one. This is useful for matching the session tempo to music which has been recorded without a metronome. Ramps may also be used as a compositional tool, but more on this later. Note that a ramp requires two points—a start and an end tempo. The first tempo in a new session is ramped, but appears to be constant as it has no tempo to ramp to. It is only when a new tempo is added and one of them is adjusted that a ramp will be heard. The same applies to the last tempo in the session—it will always appear to be constant until a new last tempo is added and changed.

A constant tempo displaying the tempo at the playhead in the audio clock — A series of constant tempo markers. The tempo at the playhead position is the same as the previous tempo.

A ramped tempo displaying the tempo at the playhead in the audio clock — A ramped tempo marker. The tempo at the playhead position is approaching the second tempo. Because the playhead is equidistant (in beats) between the two markers, the tempo at the playhead is the average of the two.

To add a new tempo, use the primary modifier and click on the tempo line at the desired position. The new tempo will be the same as the tempo at the position of the mouse click (it will not change the shape of the ramp).

To copy a tempo, hold down the primary modifier and drag the tempo to be copied.

Meter

Meter positions beats using the musical pulse of a tempo, and groups them into bars using its number of divisions per bar.

The first meter in a new session may be moved freely. It has an associated tempo which cannot be dragged by itself (although all others can). It can be moved freely and is locked to audio.

New meters are locked to music. They may only occur on a bar line if music locked.

An audio locked meter provides a way to cope with musical passages which have no meter (rubato, pause), or to allow a film composer to insert a break in music which cannot be counted in beats.

If a meter is audio-locked, its bar number is fixed from the point at which it left the main score. That bar number cannot be changed, nor can tempo motion allow the previous bar to overlap. If another bar is needed, lock the meter to music again (right click->"Lock to Music"), drag the meter to the desired bar and re-lock to audio. The new bar can be freely dragged again.

To change a meter, double click it. A dialog will appear.
To copy a meter, hold down and drag it.