Germane means relevant; it fits in. If you are giving a speech on dog training, stick to the germane, canine stuff. Topics that would not be germane? Catnip toys, hamster wheels, and the use of a saddle.
You can thank Shakespeare for the modern meaning of the adjective germane. The word originally referred to people who have the same parents. Shakespeare added the word's figurative meaning of objects being closely related or relevant when he used it in the play Hamlet. You might want to bring up all sorts of complaints during an argument with your best friend, but she says the two of you should only discuss issues that are germane to the current fight.