This question already has an answer here:
Is there a word for a term employed to play for time and allow the speaker a little longer to work out what he/she wants to say next?
Words like um and er fall into this category, of course, but so (sometimes) do terms like actually, obviously, you know, like...
I wondered about filler word, but that would cover other reasons for using such a term. It's more about the grammatical relation to the rest of the sentence (none at all) rather than the purpose for which the speaker is using it.