Signature defines the parameters and return value of a function or method in the HSL scripting language. It comprises everything following the name of the function or method, including which parameters it requires to be passed in (if any), and what it will return to a calling script (if anything).
public function usageInfo( player as NodeRef, input as String ) as String
The section "( player as NodeRef, input as String ) as String" is the function's Signature.
In practice, "signature" may also be used to describe the entire first line of a function or method, which also includes the function or method's modifier (private/public/shared/remote/untrusted).