The modulo operation is just the reminder of the division. In your example divide 88² / 187 (dont do decimals)and ignore the result but take only the remainder. 7744/187=41 remainder 77. So 88² mod 187 = 77.
In this case the modulo always returns a number between 0 and 187-1.
For example 13 mod 3 = 1 because 13/3 = 4 with remainder 1.
The modulo operator in C is the %.
I hope that I didnt confuse you. Do you need a clearer explanation?
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I may add that C=M mod n always returns C between 0 and n-1.