Slide is an archetypal move shared by Law, Lee, and Shaheen, who are often referred to as "Slide Characters." It is generally an unreactable low that knocks down on hit, side-swaps, and requires the opponent to use a specific float combo to punish.
Characters with a Slide can often form a game plan around it by mixing it up with their other moves that can be done from While Standing or Full Crouch like hopkicks.
How to do
The input is FC d/f,d,d/f+3. Slide is a move that can only be done while crouching, which means that your character must be in Full Crouch.
The command screen should be reading as:
Frame
Input
State
...
d/f
...
d
...
d/f3