These are the Lazarus sources and help files. You need to download and install FPC 2.6.4 or 3.0.0 for your platform to build Lazarus and you need the corresponding FPC sources for code navigation to work. 

fpc - the Compiler, some command line tools, base units and non visual components like database access
fpcsrc - the sources of fpc and its packages, needed for code browsing
lazarus - the IDE, visual components and help files

For an overview what changed from previous 1.6 versions
(includes incompatibilities with earlier versions) see:
- http://wiki.lazarus.freepascal.org/Lazarus_1.6_fixes_branch

For an overview what changed from previous 1.4 versions 
(includes incompatibilities with earlier versions) see:
- http://wiki.freepascal.org/Lazarus_1.6.0_release_notes
- http://wiki.lazarus.freepascal.org/User_Changes_3.0.0