/* It's used to save some memery in a struct

It basically tells the compiler that 
"hey, this variable only needs to be x bits wide,
so pack the rest of the fields in accordingly, OK?" */

// Exemple
struct remain {
  int mod3 : 2; // int usualy takes 4 bits in memory but here it will be only 2
  int mod4 : 2;
  int mod5 : 3; // and here 3
};