If you’re not familiar with layers, the best way to describe them are ‘multiple shift keys’. Keyboards already have multiple layer keys:

• shift
• alt
• ctrl
• cmd

Basically it’s a key that when pressed in combination with a second key makes that second key return a difference value to ‘normal’. As keyboards get smaller, the simplest way to accommodate things is to add additional layers - a layer for numbers, and then you can remove the dedicated number keys; a layer for arrow keys and then you can remove these too.

You then get more complex with one-shot keys, home row mods, tap-dance etc.

I’ve been using a 36-key keyboard for a year or so now, and wouldn’t go bigger. Currently waiting on a PCB to be printed that is 24 keys. Will be interesting to see how I go with that one!