CSHARP

unity key pressed

//Triggers if Space is pressed
void Update()
{
  if (Input.GetKeyDown(KeyCode.Space))
  {
    //Your code
  }
}

unity key pressed

Update is called -> GetKeyDown is true (this frame only) ->  isJumpPressed = true
Update is called -> GetKeyDown is false ->  isJumpPressed = false
FixedUpdate is called -> isJumpPressed is false 

unity on key press

// Arrow Keys
Input.GetKeyDown(KeyCode.UpArrow)

// Specific Letter e.g. "A"
Input.GetKeyDown(KeyCode.A)

// Space
Input.GetKeyDown(KeyCode.Space)