The high post in basketball is the area on the top of the key, around where the corner of the free throw line. While the high post is a little up on the court and farther away from the basket, there is also an area called the low post, closer to the rim, at the bottom of the key. Players position themselves on the high post both on offense and defense. A lot goes on on or near the high post area during a basketball game, it is an area where a lot of picks are set and a lot of pick and roll plays start. Players of all positions act on the high post, centers and forwards come to the high post to set picks, or even to post up and seek an individual play from there. 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.