Example:
IF(A1==1,"Bad",IF(A1==2,"Good",IF(A1==3,"Excellent","No Valid Score")))
The above formula means that
if A1 is 1, the result is "Bad"
if A1 is 2, the result is "Good"
if A1 is 3, the result is "Excellent"
if A1 is anything else, the result would be "No Valid Score".