The video is pretty clear, Mathew’s tutorials are very good for starting developers.
Short summary of how to use Delta:
If you would move an object 10 units over X every tick (frame), someone with 60FPS would experience 600 units per second, while someone with 120 FPS would experience 1200 units a second.
To ensure an object is moved for a fixed amount of units over time, regardless of the FPS, you multiply the amount of units you want by DeltaSeconds.