“Implicit reference semantics” means that variables ordinarily refer to objects rather than containing them. “Value semantics” means that variables contain values rather than references, though there are pointer types that let you explicitly store references when you want them. (Often this is discussed in terms of parameter passing, for historical reasons, but the same ideas apply to variables more generally.)

If your language has implicit reference semantics, “x = y” will cause x and y to refer to the same object. If it has value semantics, x will be a copy of y.