What do this ":" does

Question

I was looking at some c code..Where i saw this:

struct bg_cfg
{
u8 bg_id:2;
u16 charbase:2;
u16 screenbase:5;
};
So , what actually : in bg_id:2 is for?

NOTE: u8 , u16 is unsigned char and unsigned short int respectively.

Answer 1

found the answer. BITFIELD..

https://www.tutorialspoint.com/cprogramming/c_bit_fields.htm