Very commonly an application within a symfony project. Often existing solely by itself; other times existing in tandem with a backend application. It is not required of a project to have one, it is only a common convention to use for the main front end shown to the user.
listing from Jobeet:
> ls -l apps/frontend drwxr-xr-x 12 user staff 408 Jan 12 14:12 config drwxr-xr-x 3 user staff 102 Dec 8 16:50 i18n drwxr-xr-x 4 user staff 136 Jan 7 15:24 lib drwxr-xr-x 7 user staff 238 Jan 12 13:40 modules drwxr-xr-x 5 user staff 170 Jan 12 13:47 templates