An audit trail is like a storybook, but instead of telling a story about a princess or a superhero, it tells the story of how someone used a computer or a system. Just like a princess needs to make sure that her castle is safe and secure, the people who make computer programs want to make sure that their programs are safe too.
So, they create an audit trail to keep track of all the things that happen in their programs. Every time someone does something in the program- like logging in, changing something or accessing a file, the audit trail notes down what was done, and who did it.
This way, if anything goes wrong or if someone is doing something they shouldn't be doing, the people in charge can follow the audit trail like breadcrumbs to find out what happened and who is responsible.
Think of it like a detective story- the audit trail helps the detectives (in this case, the people in charge of the computer program) to figure out who did what and when they did it. Just like detectives need clues to solve a mystery story, the audit trail gives the people in charge of the program clues to figure out what's happening behind the scenes.