Off the glass in basketball is a term that refers to a situation where the basketball hits the backboard and bounces off. The backboard is made of glass, so using this terminology is another way for an announcer to say 'off the backboard'.
While a shot could be described as off the glass even if it misses the basket, this phrase is typically used after a player successfully makes a shot off the backboard.
The backboard is part of the basketball hoop structure. Regulation backboards are 72 inches wide and 42 inches tall.
A backboard shot can be useful in many different scenarios. The most common off the glass shot is a layup. Players can toss the ball up and off the glass from in close for an easy basket.
Long range backboard shots are much less common in modern basketball. One reason for this is that a backboard shot is not as flashy as a swish.