It sets bitmasks:
1U << 0 = 1
1U << 1 = 2
1U << 2 = 4
etc...
What happens is 1U (unsigned value 1) is shifted to the left by x bits.
The code you posted is equivalent to:
enum
{
IsDynamic = 1U, // binary: 00000000000000000000000000000001
IsSharable = 2U, // binary: 00000000000000000000000000000010
IsStrong = 4U // binary: 00000000000000000000000000000100
}