A poll done by a campaign for the campaign's own internal use.
They are sometimes released to the public (if the results are favorable to the candidate).
The last part is important. Internal polls aren't inherently either better or worse than polls done for the media, even though some people sometimes tend to view internals are either better (i.e. the Romney people in 2012) or worse than those.
The sole reason why we sometimes tend to dismiss internal polls is exactly because we only tend to see them when they produce good results for the side they represent.
So all polls have statistical uncertainty. The problem is if an internal operation do two polls that are both say 3%-points off, we only get to see the poll that are off in the desired direction leading to a scewed view of where the election is at. This is also why you might just assume that an internal poll being released is probably 2-4 points too generous to their candidate.
