Free Pascal is under continuous development. From time to time, a new set of installers with what are considered stable sources are made: these are the releases. They can be downloaded from the Free Pascal website. The downloads usually contain the sources from which the release is made.
If for some reason, a newer set of files is needed - for instance, because certain bugs that prevent a program from functioning correctly have been fixed, it is possible to download the latest source files and recompile them.
Note that the latest sources may or may not compile: sometimes things get broken, and then the downloaded sources are useless. For the fixes branches (mentioned below) the sources should always compile, so it may be best to use these only.
There are 3 ways to get the latest version.