Something I like to think I am.. <great dad joke!>
A stud is the circle on the top/side of LEGO bricks. It is a key mechanism for bricks connecting to each other. It is also used as a way to quickly reference the part type.
For example, a 2 x 4 Brick would be a brick that has a grid of 2 x 4 studs. This brick is shown below. Similarly a 2 x 2 brick would be 1/2 the size of this brick.