M-expression is short for "Malicious Expression." "Malicious" means something that is meant to harm you, like a bully who hits you or a virus that messes up your computer. "Expression" means putting something into words.
In computer programming, an m-expression is a type of code that is meant to harm your computer or steal your information. It looks like normal code, but it does bad things, like deleting all your files or sending your emails to someone else.
It's important to be careful when you download or use any programs or files you don't know about because they might have m-expressions in them. Always scan your files with antivirus software and make sure you trust where you're getting them from.