In golf, a birdie simply means one under par. Each hole is given a par. Most courses consist of par 3s, 4s, and 5s. A birdie on a par 3 would be a score of 2. A birdie on a par 4 is a score of 3. A birdie one a par 5 is a score of 4. A birdie means you got the ball in the hole in one less stroke than the par given for the hole.
