Layers — keyboard feature used to assign multiple functions to one key. To understand what is layer remember Fn-key on notebook keyboards: pressing it usually changes meaning of F1..F12 functions thus «opening Fn-layer». Another example of layer is Shift key: pressing key  without Shift sends symbol «1», while pressing key  with Shift sends symbol «!». So you can say that «!» is on second layer.
Mods (Mod-keys) — Alt, Ctrl, Shift and Win keys.
6NKRO — keyboard hardware ability to capture simultaneously at least 6 pressed non-modifier keys; most mechanical and gaming laptop keyboards do have it, while most membrane keyboards usually have only 2NKRO. Non-registered keys are referred to as ghosted.
Keyboard sizes (100% keyboard, 60% keyboard, etc.) are best explained with picture.
Mechanical keyboard — keyboard that uses mechanical swithces. These switches are usually louder than membrane switches, but have (arguably) more comfy feeling. They are often expensive and there is a whole world of creating custom mechanical keyboards.
Membrane keyboard — keyboards that use membrane switches. Traditional $10 keyboards are those. They are usually cheap, has only 2NKRO, don’t produce much noise. Laptop keyboards are usually membrane too. Gaming laptops (starting from $1000) usually have 6NKRO membrane keyboards.
QMK (Quantum Mechanical Keyboard) — a keyboard firmware very popular among mechanical keyboard enthusiasts. Used to code complex keyboard behaviour for custom keyboard (like layering for example).
Pro Micro — one of the most popular controller used to create custom keyboards. Well documented and compatible with QMK.