The general idea is that the set of all contingent things can only be explained by something non-contingent (ie necessary). A necessary thing, by definition, i

The general idea is that the set of all contingent things can only be explained by something non-contingent (ie necessary). A necessary thing, by definition, is its own explanation. "I am what I am" etc. From there, classical theists attempt to connect that necessary thing to what you'd commonly understand as God.