What copyright covers, and what it does not
Copyright protects original literary, dramatic, musical and artistic works, together with cinematograph films and sound recordings. Software is protected as a literary work, and so are databases and compilations where there is originality in the selection or arrangement.
What copyright does not protect is ideas, facts, methods or concepts: only the particular expression of them. A business idea is not protectable; the specific text, code or design in which you expressed it is. Brand names belong under trademark, and inventions under patent.
Registration, and why it is worth doing anyway
The application is filed with the Copyright Office with the particulars of the work, the author and the owner, along with copies of the work. There is a waiting period during which objections may be filed, and where none are received the registration is granted and entered in the Register.
Registration matters most at the moment of dispute. A registered certificate shifts the practical burden in litigation, satisfies platforms processing takedown requests, and is frequently required in commercial transactions where copyright is being assigned or licensed.
Enforcement
Infringement remedies include injunctions, damages or an account of profits, and delivery up of infringing copies. Copyright infringement can also be a criminal offence, which gives an additional route where commercial-scale copying is involved.
For online infringement, the intermediary takedown process is usually faster than litigation. A registration certificate makes those requests materially easier to get actioned.