I'm trying to understand transaction malleability in Bitcoin (before SegWit solved it).

Every explanation of it reads the same and basically says "Bob alters the signature of Alice's transaction before it gets confirmed, thus producing a new transaction ID".

But what does changing a transaction actually mean? How do I approach that? Saying you grab a transaction from the mempool and flipping its bits really does not provide the explanation for me.

Looking forwards to your replies