This command subtracts a value from a variable. You should NOT write the keyword LET before the variable. LET is not a valid keyword in ScriptBasic.
The variable can be a global or local variable, array element or associative array element.
You can use this command as a shorthand for v = v - expression. Using this short format is more readable in some cases and generates more efficient code. However note that this kind of assignment operation is a C language like operator and is not common in BASIC programs.