NVL(exp1, exp2) and NVL2(exp1, exp2, exp3) are functions which check whether the value of exp1 is null or not.
If we use NVL(exp1,exp2) function, then if exp1 is not null, then the value of exp1 will be returned; else the value of exp2 will be returned. But, exp2 must be of the same data type of exp1.
Similarly, if we use NVL2(exp1, exp2, exp3) function, then if exp1 is not null, exp2 will be returned, else the value of exp3 will be returned.