Reading the world.
A block is a place with a kind: the block below a player, the block they look at, the blocks of a kind in a box or on a layer. Chunks are the game's sixteen-by-sixteen columns, and the loaded chunks and the chunks near a place are lists a step can walk.
Changing it.
Set a block, break one with or without drops, fill, replace or clear an area, set a biome. Big changes are queued and applied a slice a tick, so the server stays smooth; what is still waiting shows in the status.
Areas.
An area is a box between two corners or around a place, and a layer is one height across the loaded chunks. Walking every block of a huge box is work the engine counts, so keep a box to what the rule needs.