what does 'erm' mean?

It is the equivalent of 'um', 'oh', 'uh' or anything else that is sort of a sound you would make when you are caught off guard and need to do something unexpected, and you make a sound while you are doing it.

 Customs: Can I see your passport please?Peter: Oh! errrrrrrm, here you are. (rummages in carry on bag for his passport, saying 'erm' while he does it.)

a noise that people make which means nothing at all except that they are taking more time to come up with what they intend to say and don't want to leave a big pause.  It is a filler noise to indicate "I am not done talking yet". SO, I guess that is what it means, in a way "I am not done talking yet even if I have stopped making new words at the expected rate".

This Is the throat clearing noise. But said in a written form. Same way 'ha-ha-ha', Is laughter in written form.

Sometimes even good orators will do this unconciosly, when thinking of something to say

It is a 'filler', like 'um'. It has no meaning. It simply fills a moment of silence and allows the speaker time to think.  Another favourite filler these days is 'like', as in: "I was, like, surprised!"