Basically, many cryptographic hashes support fixed-length hashes of variable message lengths by breaking the message into blocks, chaining their hashes* and taking the final hash.

The weakness here is if you know the length of a prefix and its hash, you can generate more valid hashes of messages that contain the unknown prefix but with custom suffixes. This is relevant if you use the hash for authentication (i.e. MAC) as it allows producing certain types of custom messages that would also validate.

However, this has largely been a non-issue for a long time now as it takes very little tweaking of the protocol (stuff being authenticated) to make adding suffixes useless. Double hashing is one such mitigation, because the outer hash is now working over a fixed size input, meaning to attack it you'd need to the signed message instead of just appending to it.

*: This approach of chaining hashes of blocks is also used in other contexts that you may be familiar with ;)