It means "left shift" in binary. This corresponds to adding a specified number of zeros at the end of the binary representation of the original number. So 1 << 0 is binary 1, 1 << 1 is binary 10, 1 << 2 is binary 100, 1 << 3 is binary 1000...
If you think about the meaning of place value in binary, this is exactly the same as raising two to a specified power. Each time you shift one place further left in binary, it's equivalent to multiplying the existing number by two. So repeating that a specified number of times is multiplying by a specified power of two.
If you think about the meaning of place value in binary, this is exactly the same as raising two to a specified power. Each time you shift one place further left in binary, it's equivalent to multiplying the existing number by two. So repeating that a specified number of times is multiplying by a specified power of two.