A birdie is a term used to denote a score of 1 under par on a single golf hole.
It is important to note that you must shoot exactly 1 under par in order for it to be called a birdie. Nevertheless, you can still make a birdie on all 3 types of holes in golf: Par 3s, Par 4s, and Par 5s. A golfer must complete the hole in 2 strokes on a Par 3, 3 strokes on Par 4, and 4 strokes on a Par 5 for it to be a birdie.
Because the term "par" is the expected or average amount of strokes needed to get the ball in the hole, a birdie is viewed as an above average score. It is not uncommon for an experienced player to score numerous birdies throughout a round of golf. However, casual players may have a tough time reaching this benchmark given that you have to be extremely good to shoot par in the first place. In other words, the predetermined "average" amount of strokes greatly relies on the players involved in the game. For some, the course may be easier than usual, and for others it may be much harder.