`declare` - declare variables and give them attributes

declare [-i] [-a] [-A] [-r] [-x] [-p] [NAME[=VALUE]]

Options you will use

-i: Integer: assignments are evaluated arithmetically, so counter+=1 adds instead of concatenating.
-a / -A: Indexed array / associative array.
-r: Read-only.
-x: Export.
-p: Print the declarations.
-n: A nameref: the variable is another name for the variable named in its value.
-f: Functions: print function definitions.
-g: Create a global variable, even inside a function.

Try declare in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.