I've only been with TAXI for about 2 weeks and was amazed last night to receive 2 emails saying I'd had some music forwarded! This is fantastic but I now realise that I need to start understanding how the business actually works

This is what the listing says -
EXCLUSIVE deal, the music you submit for this pitch CANNOT be signed with any other publishers or catalogs. This Exclusive, In-House Library will pay an initial buyout fee of $250 for 100% of the Composition and Master rights. On top of the per track fee, you will also keep 100% of the Writer’s share of the publishing and make applicable performance income.
Would anyone be able to explain this to me or point me to any good sources of online information about this subject? Some of it is obvious but what would this mean in practice if it gets used once/multiple times etc?
Also, do people copyright everything they produce? Is it worth copyrighting short cues or would you only do that with full songs?
Sorry for the newbie questions!
Thanks in advance
Mark