A high post in basketball is the area on the court at the elbows of the key by the free throw line. A high post offense refers to at least one player standing in the high post area and receiving a pass. Usually, this means one forward or center is in the high post and the offense will flow through them. High post plays will usually have the word "high" worked into the title.
A low post in basketball is the area on the court by the block, near the basket. An offensive player will be in the low post and the offense will flow through this player in the low post. As with high post plays, a low post play will often be indicated by the word "low" somewhere in its title.
Coaches often combine high- and low-post concepts to create more structured offensive movements that allow for greater options. These are called high-low post plays.
Designing offenses around a high or low post can confuse the defense by focusing the collective attention of the defense onto one part of the floor. The offense will present opportunities for the high- or low-post player to score themselves or pass to teammates, based on the specific actions of the play. These often involve screens, handoffs, or cuts near the post-up player.