The A button is the primary face button on the Nintendo 64 controller, and is widely used throughout Super Mario 64. Completing this game without pressing the A button, or failing that, pressing it as few times as possible, is the core goal of the Super Mario 64 A Button Challenge.
The A button is primarily used to make Mario jump. This includes Mario's basic Jump, as well as moves such as the Double Jump, Triple Jump, Wall Jump, Side Somersault, Backflip, and Long Jump. It is also used for contextual actions such as scrolling through dialogue, swimming, hanging from ceilings, and jumping from poles; as well as combo moves such as the Jump Kick, Midair Dive, and Ground Pound. Some of these moves can be activated through other means and buttons, and there are differences between releases of the game that may alter these means and therefore incur new potential A presses depending on version.