Meaning of U suffix

It stands for unsigned.

When you declare a constant, you can also specify its type. Another common example is L, which stands for long. (and you have to put it twice to specify a 64-bit constant).

Example: 1ULL.

It helps in avoiding explicit casts.

Integer constants in C and C++ can optionally have several suffixes:

123u      the value 123 is an unsigned int
123l       (that's a lowercase L) 123 is a signed long
123L      ditto
123uL    unsigned long
123LL    a signed long long, a 64 bit or 128 bit value (depending on the environment)
123uLL  unsigned long long