What is a down in NFL football?

Answer 1

You get four chances (downs) to advance the ball 10 yards. If you get the 10 yards then you get 4 more "downs" to work with. However, if you fail to advance 10 yards then the opponent gets the ball at the point of your last play (except in the case of a punt). Typically, if a team does not get the 10 yards after the 3rd down then they will punt on the 4th down in order to make the opponent start further back.

Answer 2

Loss of down is a tacked on penalty. Most common use of it is with intentional grounding, the offending team loses yards and the down they just played.