One under par for a hole is one shot less than par, say you make a 3 on a par 4, this is a birdie. One under for a round is where you had a round which was one shot less than par, say you shot 71 on a par 72.
For holes played, or for that round, or for the tournament, the player has taken one less shot to play those holes than par.
one under par for that hole is par 4 and you get 3
A par is a certain ammount of strokes you hit the ball with in golf. If you hit under the ammount of par it is reffered to as an under par. Which is different depending on the difficulty.
A birdie is a golf hole played one under par.
two under par
A birdie is one stroke under par on a given hole.
5 under par
One under par in golf is called a bogey.
One under par on a hole is called a "birdie".
Three under par on one hole of golf is an albatross or a double eagle.