Delta-time is a measure of the difference between frame timings.
So if you were to have a RenderStepped function your delta-time would be equal to the time between the current frame and the previous frame. This is useful for calculations that are dependent on the client’s framerate.
Delta just means “the difference between”. So delta time would refer to the amount of time that’s passed since some event. In the context of running code on every frame, for example, delta time refers to how much time has passed since the previous frame.
Based on my experience on using delta time on other coding languages, it’s when you have something running on the same speed no matter the framerate. (I could be wrong though.)