A 'low block' in basketball is one of the two rectangles on the court located on the sides of the painted area near the basket. They are used a reference point for certain play calls, providing offensive players with a visual target to stand on.
One of the arguably most essential aspects of maintaining success on offense is getting position on one of the blocks. The offensive team's power forward or center will often be instructed to stand near the block, as these players are generally the tallest athletes that each team has to offer. This allows them to make a series of moves to fake out the defender, before scoring the ball from close-range.
'Establishing position' requires the offensive players to use their bodies to block out defenders. Once an offensive player has managed to gain separation and force the defender under the basket, the ball handler can pass the ball into the painted area, allowing the offensive player to attempt an easy shot with a high make probability (i.e. a layup).
In order to prevent an offensive player from gaining position on the block, defenders often use a tactic known as 'fronting '. Rather than remaining stationary and waiting to defend the player until the ball is entered into the painted area, defenders can lean around the player and extend a hand out front. The outstretched hand will either deter the ball handler from making a pass in the first place, or allow the defender to come away with a steal if a pass is attempted.