Utility is the amount of satisfactions(it could also be negative).
Say you are very hungry and then your friend gives you a biscuit, because you are hungry we can say the number of utility is 10 (assuming that 10 utils is the maximum).
Say that you already ate few biscuits and your mom gives you a piece of cake, you accept it just to be polite and the utility will be 1.
After eating the cake your other friend come with peanuts chocolate(assuming you are allergic to nuts), it makes you sick and you give it -4 utils.
Utility is not a realistic measure but it gives us a clear indication on the consumer decision on the products.