Copying and linking are two different methods of using existing audio files on the computer (or network file system) within a session. They differ in one key aspect:

Copying

An existing media file is copied to the session's audio folder, and if necessary converted into the session's native format.

For audio files, the format can be chosen (e.g. WAVE or Broadcast WAVE). Audio files will also be converted to the session sample rate if necessary (which can take several minutes for larger files).

MIDI files will already be in SMF format, and are simply copied into the session's MIDI folder.

Linking

A link to an existing media file somewhere on the disk is used as the source for a region, but the data is not copied or modified in any way.

While linking is handy to conserve disk space, it means that the session is no longer self-contained. If the external file moves, it will become unavailable, and any changes to it from elsewhere will affect the session. A backup of the session directory will miss linked files.

The Copy file to session option in the Import dialog window allows to choose to copy or link files into the session:

There is a global preference Edit > Preferences > General > Session > Always copy imported files. If it is enabled, linking a file will not be possible.