Everything contained inside a bracket pair prefixed by a $ (like $[this]) is considered as an expression and it is evaluated. Evaluation works similar to (but is done on a later stage than) variable substitution: the expression (including the square brackets) is replaced by the result of the expression evaluation.
For example, after the sequence:
the value of variable koko is "6".
and, further:
will parse as intended. Extra spaces are ignored.
Page: Spaces Inside Variables Values
Page: Operators
Page: Floating Point Numbers
Page: Expr2 Built-in Functions
Page: Expressions Examples
Page: Numbers Vs. Strings
Page: Conditionals
Page: Expression Parsing Errors
Page: NULL Strings
Page: Warnings about Expressions
Page: Expression Parser Incompatibilities
Page: Expression Debugging Hints