The core of it is that it means a malicious document will pass the check of authenticity when you are using the genuine signature. Someone could tamper with a m

The core of it is that it means a malicious document will pass the check of authenticity when you are using the genuine signature. Someone could tamper with a mirror infect you that way. Absolutely no tampering with the signature would be required, which is what makes this dangerous. Basically, it can be used to send to entirely fake data that you can't tell is fake. Or someone could say that you have your legally binding signature attached to a contract with hash ABC, but then present a different contract with hash ABC but very different terms. Those are the end state. This is a major step closer to that end state. Neither of those scenarios are the case today, but they're now close enough that it's a matter of time. And likely not a lot of time. Time to abandon SHA1.