Yes, it means afterward or afterwards. It can also mean subsequently, later, hereafter, at a later time, or later today of course depending on the context. And bis nachher means "till then or until then". Hope this helps.

nachdem is also a conjunction, "after".

For example, nach dem Regen "after the rain" but nachdem es geregnet hatte "after it had rained".

The difference is that the preposition nach stands before a noun or noun phrase but the conjunction nachdem stands before an entire clause (with a verb in it).

You can't say nach es geregnet hatte for "after it had rained".

nach + noun, f.i. "Wir gehen nach dem Essen!"

nachdem + sentence, f.i. "Wir gehen, nachdem wir gegessen haben!"

nachher is used when everybody knows what the "after" refers to, f.i. "Wir gehen nachher!"