Syntax

Syntax

forexample in Mozilla Developers its written:

Syntax

arr.forEach(callback[, thisArg])

i know forEach usage etc. but i dont understand the way it declares the syntax
its written like
[, ]
what does
callback[, thisArg] mean?
why it opens the bracket, puts comma and closes bracket after
thisArg ???
in fact i cant learn forEach syntax and usage by the way its declared

8-}