git.git: topics and things like that

Hi,

This may appear as a stupid question but as I can not find
anything about this, I am just asking.

So, how is git development organised ? I saw there are topics but
I do not know how patches are applied mainline (onto tip of
master).

Would you mind explaining this very quickly so I can understand
messages "What's new in git.git" ? :)

Thank you very much and sorry to bother with this basic question.

Xavier