The ':' operator

What exactly the ':' operator does?

Code:

---

struct SByteBits
{
    BYTE b7 : 1;
    BYTE b6 : 1;
    BYTE b5 : 1;
    BYTE b4 : 1;
    BYTE b3 : 1;
    BYTE b2 : 1;
    BYTE b1 : 1;
    BYTE b0 : 1;
};

---

https://msdn.microsoft.com/en-us/library/yszfawxh.aspx

Great, thank you!