Chapter 2-Why use bitwise and with crc32

Hi,
I'm not a pro to python and I am wondering why we use bitwise and on page 52.

```python
def test_set_check(identifier, test_ratio):
return crc32(np.int64(identifier)) & 0xffffffff < test_ratio*2**32
```

could someone explain me the purpose of this?
Thank you.