The top of the key in basketball is a location on the court referring to the topmost point of the arched line that connects both sides of the free throw line. The rectangular, painted part of the court (the paint) with the semicircle at the top is known as the key. The top of the key is the space that is just in front of the three-point line and just past the free throw line. This is a popular shooting spot, so it's common in zone defenses to have a player cover the top of the key.
The court in basketball has gone through numerous changes throughout the sport's history. Specifically with the "key", there have been several increases and decreases in regards to the height and radius of the circle/semicircle. The first iteration of the key was much larger than what it looks like today.
Even years later, there is no standard for how large the key should be, with it varying in different leagues (NBA, NCAA, and FIBA). The NBA's key is 16 feet in diameter, the NCAA's is 12 feet, and FIBA's is also 16 feet but does not have the full circle.
Due to the introduction of analytics within the sport players have strayed away from attempting a lot of shots from the top of the key. That is because it is the most inefficient shot a player can take. Out of all two point attempts it is the furthest spot away from the basket. Three point shots are further but are worth a full point more. Shots from the top of the key yield about 86 points per 100 possessions. That is far below the number for corner three pointers (119 points per 100 possessions). Due to that difference being uncovered teams have had their players take less shots from the top of the key in today's game.