Frame Walking is a glitch that allows Mario to walk up steep slopes that would normally cause him to slip and slide.
It is done by alternating between no input on the joystick and using the joystick to walk up the slope. The optimal method is 1 frame no input, and 1 frame walking up the slope. However it is not frame perfect, and multiple frames can be taken with each of them. It's just slower.
Frame walking does not work on slippery slopes, which cause Mario to slide down even if frame walking is attempted.