The zoom controls allow to navigate the session along both the time and track axes.
The drop down Zoom Focus menu allows to select a focus point for the zoom, i.e. the center of the zoom. The choices are:
The 2 leftmost zoom buttons (− and +) use this zoom focus to zoom out and in respectively.
The Zoom to session button is a handy shortcut to zoom out or in until all the session (as defined by it's start/end markers) fits horizontally.
Changing the Number of visible tracks dropdown menu allows to fit this number of tracks vertically in the screen.
There is a minimal track height to keep it visible, so according to you screen vertical size, some high number can have no effect.
Inside this menu are 2 handy choices:
The rightmost buttons Shrink tracks and Expand tracks reduce or expand the vertical size of the selected tracks. If no track is selected, all the tracks will be shrunk or expanded each time the button is pushed.