Hotbar
The hotbar is used to quickly select one item of some of the items from the player inventory. It is usually located at the bottom of the screen and consists of 1 to 23 slots.
The currently wielded item is somehow emphasized in the hotbar. The slots are numbered sequentially, beginning from 1. A hotbar slot of the corresponding slot number can be selected directly by pressing one of the number keys 1 to 9. The key 0 selects slot 10. Mouse wheel up selects the next, Mouse wheel down selects the previous slot.
Usually the first n slots in the player inventory make the slots of a hotbar of size n.
Default hotbar
The default is located at the bottom of the screen and holds 8 items. The top line of the player inventory makes the hotbar. The red square marks the wielded item. An empty slot selects the hand.
The default hotbar is used in Minetest Game and in any game which does not specify it otherwise.
Hotbar in other games
Other Minetest games may choose a different size of the hotbar and a different position in the inventory, for example, the bottom line instead of the top line and even a different design.
But most games just stick with the default hotbar at the moment.