blk*.dat format is a repetition of:

- magic (uint32-le)
- size  (uint32-le) - bytes count of following block
- block (https://en.bitcoin.it/wiki/Protocol_specification#block)

The block spec points out that `bits` is a uint32-le.

`bits` represents difficulty in compact form and you can see how to expand it here https://en.bitcoin.it/wiki/Difficulty.


The wiki (https://en.bitcoin.it/wiki/Protocol_specification) is pretty comprehensive.