A project consists of basically two things: Your source files, and the project data files that Source Insight maintains.
Figure 3.1 The components of a Source Insight project.
Each project contains a list file paths that point to your source files. A project also contains a symbol database, which is maintained by Source Insight. Your source files are parsed by Source Insight and symbol declarations and definitions, and references are recorded in the symbol database. All you have to do is add source files to your project; you do not have to generate any other "tag" files. Source Insight does that automatically.
Each project has its own session workspace. The workspace contains session information, such as the list of files that are open and window positions.