A filler, basically, usually added to the end of a sentance. Stereotypically said to be incessantly used by Canadians, just like everyone thinks all American teenaged girls with blonde hair say 'like' repeatedly, only 'eh' isn't so incredibly obnoxious IMO. Personally I've lived with canadians that say it, others that don't. Depends on the canuck. Plenty of Americans say it too. Can also be likened to the habit of adding 'then' to soften a sentance.
'so you see what I'm saying, eh?'
'you want to go to the mall, then?;