The high post in basketball is a location on the court where players position themselves on both offense and defense. It's right at the top of the rectangular area of the court that's known as the key, located just on the edge of the free throw line. There is also a location on the court called the low post, which is closer to the basket, at the bottom of the key.
This is an area of the court where players set a lot of picks or screens on offense to try to get their teammates open for a shot. You may see a center or forward come to the high post to retrieve the ball and make a pass since they're only allowed in the key for three seconds at a time. Guards take a lot of shots from the high post at the corner of the key, called the elbow. At the high post, an offensive player has a number of options. He's close enough to shoot a decent shot, he could pass to the low post for a closer basket, he could drive to the basket or he could pass it back out to an open teammate. All of these options make the high post a popular spot on the court and at times difficult to defend.
On defense, you may see several players position themselves at the high post, especially in a zone defense. For example, in a 2-3 zone defense, the two players at the top will stand at the high post with their arms spread out. In a 3-2 zone, one of the three players at the top will defend the high post and the other two will be closer to the wings. In a 1-3-1 zone defense, the three players in the middle will be spread across the high post. Since this spot is such a common passing and shooting area, many defenses are sure to cover the high post. This area is also a direct path to the basket, so it's important for defenders to block that path.