Definitions for "Slowroll"
To reveal one's hand slowly at showdown, one card at a time, is to slowroll anyone else who thinks the pot might be theirs. This is usually only done with a winning hand, for the purpose of irritating other players (well, some people do it innocently).
To await showing a winning hand longer than necessarily.
To reveal one's hand very slowly at showdown, with the express purpose of heightening drama.