What is the function displays the highest value in a range?

This would be the MAX function.

=MAX(A1:A12)

You can also use the LARGE function, with 1 specified as the position to find. 1 specifies the largest, 2 the second largest, 3 the third largest and so on. LARGE is best used when it is not the highest value you are looking for, but it still can be used to find the largest. It can be done like this:

=LARGE(A1:A12,1)