what does this mean in c int a:16;?

This is a bitfield.

This particular bitfield doesn't make much sense as you just could use a 16-bit type and you're wasting some space as the bitfield is padded to the size of int.

Usually, you are using it for structures that contain elements of bit size:

struct {
    unsigned nibble1 : 4;
    unsigned nibble2 : 4;
}