Integer literals like 1 in C code are always of the type int. int is the same thing as signed int. One adds u or U (equivalent) to the literal to ensure it is unsigned int, to prevent various unexpected bugs and strange behavior.
One example of such a bug:
On a 16-bit machine where int is 16 bits, this expression will result in a negative value:
long x = 30000 + 30000;
Both 30000 literals are int, and since both operands are int, the result will be int. A 16-bit signed int can only contain values up to 32767, so it will overflow. x will get a strange, negative value because of this, rather than 60000 as expected.
The code
long x = 30000u + 30000u;
will however behave as expected.