Mini-LED
Mini-LED is a backlight made of a very large number of tiny light-emitting diodes. It makes LCD TVs and displays brighter and higher in contrast.
What does Mini-LED mean?
An LCD display doesn't emit light itself, it needs light from behind. With Mini-LED, this light comes from thousands of small LEDs that are dimmed individually in zones. Dark areas of the image become darker, while bright areas stay bright.
Compared with OLED, Mini-LED usually reaches a higher brightness. OLED keeps the advantage with black, because there every single pixel switches off. With Mini-LED, a slight halo can be visible around bright objects on a dark background.
A general explanation of the term, not from a video. How it plays out on a specific device in everyday use is covered in the reviews below.