By default Lazarus-IDE only opens one project at a time, and that's not always nice. In some environments it is convenient to have all projects in a group and edit/compile whatever is most convenient. For example, opening all projects in a group, and compiling all of them in one move, instead of compiling one at a time.
If you think so too, go to Package|Install/Uninstall Packages and install the package “lazprojectgroups”:
After adding the package go to “Save and rebuild IDE”, after restarting Lazarus you will notice this component in the Project|New project group menu to create a new group or Project|Open project group to open an existing group. You can dock it on a side panel if you want to make it permanent.
To learn the basics of its use, see This article.
Or watch this video to see how it works: