Server discussions are full of terms that get used as if everyone knows them. Here is what they actually mean, in the order you are most likely to meet them.
The core vocabulary
One pass of the server's world simulation. It aims for twenty per second, one every 50 milliseconds.
Ticks per second. Twenty is healthy. Below twenty means the server is falling behind and the game runs slowly.
Frames per second, on the player's machine. Nothing to do with TPS, and confusing them wastes hours.
A 16 by 16 column of the world, full height. Everything loads, saves and ticks in chunks.
Anything that is not a block: mobs, dropped items, boats, arrows, item frames.
A block with data attached — chests, signs, hoppers, furnaces. More expensive than plain blocks.
Operator. The highest in-game permission level, granting every command.
The Java program file that is your server. paper-1.21.jar is a jar.
An address and port your service is allowed to listen on. Your game port is one; extras exist for things like a web map.
A list of accounts permitted to join. Everyone else is refused at connection.
Message of the day — the two lines shown under your server in the multiplayer list.
Server-side code for Paper or Spigot. Players install nothing.
Code that usually runs on both server and client. Players need the same set you have.
Vanilla-supported data files that change recipes, loot and functions. Lives inside the world folder.
The number that determines how the world generates. The same seed makes the same terrain.
A server that sits in front of several servers and makes them look like one address.
TPS and chunk. Almost every performance conversation is about how many chunks are loaded and whether the tick fits in its 50 milliseconds. Everything else is detail on top of those two.