The name somewhat gives the game away here. Mini-LEDs use very small LEDs to produce the display’s light. It is a new type of backlight tech for LCD displays and it matters because it’s a very good improvement over current LCD backlight technology while also not being too expensive. This new technology traces its roots back to traditional backlit LCD technology. Except rather than using a single large or multiple smaller locally dimmed backlights, Mini-LED uses thousands of tiny LED backlights to offer vastly superior local dimming characteristics. To meet the Mini-LED classification, these backlight diodes measure less than 0.2mm across each.
The new Apple iPad Pro 12.9" uses 10,000 Mini-LEDs vs... compare to just 72 on the previous generation model
These 10,000 individual Mini-LEDs are then organized in about 2,500 "local dimming zones", which allow for that super high contrast.
Mini-LED moves to thousands rather than hundreds of tiny backlights allows for deeper blacks, improved contrast ratios, and brighter panels. That’s great for HDR content. All thanks to the smaller components. Mini-LED technology also scales from small to huge panel sizes quite easily, as there’s no meaningful limit on the size and density of the backlights.
Mini-LED is not to be confused with Micro-LED.
Micro-LED was first introduced back in the year 2000 but it does share the same principle as Mini-LED, only smaller. If a regular backlight LED is 1mm across then a mini LED is 0.2mm while a micro LED is 0.005mm in size.
Mini-LED has the benfit of costing less but this also means it's not as good as Micro-LED tech in terms of local dimming performance.
In conclusion, Mini-LED is a new technology with costs way cheaper than OLED screens but just about the same in terms of quality.