Please explain the concept of mutability. In the program Snap! (which we use at Berkeley), there are things such as immutable objects (e. numbers,...

Please explain the concept of mutability. In the program Snap! (which we use at Berkeley), there are things such as immutable objects (e.g. numbers, texts) and things such as mutable objects (e.g. lists). And according to class notes, it says that "If objects existed outside of this procedure (i.e. not a script variable), then this is a side effect." What exactly are side effects in this case? I don't understand this concept.