eproject is an Emacs extension that adds the concept of projects to Emacs. The eproject core adds two functions for lisp programs, (eproject-name) and (eproject-root); the eproject-extras package uses this functionality to provide easy navigation between project files, add project information to ibuffer, project-local variables (per-project indentation style, for example), and some other things. Most people will want to install both, and most elisp programmers will want to read through both files to get ideas on what functionality is available.
If you haven’t installed eproject yet, read InstallingEproject. Once you’ve gotten eproject installed and loaded into your emacs, type `M-x customize-group eproject` to see what preferences are available. Then you can steal some eproject configuration from the hive mind by reading EprojectExamples!