Front end contains your forms, queries, reports, macros, modules and VB code.
Back end only has tables. Tables store the data.
When you split a db, you are creating 2 files. Both are Access databases. You the tell the FE where the tables are to be found. This is called linked tables. Once a table is linked, the FE "sees" them as being part of the FE rather than another file.
The BE could also be made up of more than one file.