data type
A data type is what KIND of value something is. Is it text? A number? A simple true/false? A list of things? The type tells the computer — and you — what a value really is, and that decides what you're allowed to do with it.
The common ones show up everywhere: a string is text like "hello", a number is something like 42 or 3.14, a boolean is just true or false, and an array is an ordered list. Each comes with its own toolbox of things it can do.
Type matters because it sets the rules. Add two numbers and you get their sum; 'add' two strings and they join end to end. Try to multiply a word by a word and the program simply can't — that's the type quietly keeping you honest.
typeof "hello" // "string" typeof 42 // "number" typeof true // "boolean"
Ask any value what type it is — text, number, or true/false.
Some languages check types up front (typed), others sort it out as they run (dynamic) — same idea, different timing.