They are fruit flies.
They are either coming from your fruit basket (attracted to any soft part of fruit) or brown bin.....once one or 2 get in there, they can multiply very quickly.
Another option (especially if you have kids) is they may have left a piece of fruit (part or whole)....somewhere.
We had a major problem a number of years ago, turned out one of the kids had put some bananas into an old sewing basket which was never used. Took us a while to track it down.....but easy to sort once you get to the scource