Congress may not pass ex post facto laws. Explain why?

Congress may not pass ex post facto laws because for example lets say that marijuana was legalized in the 80’s and in the 90’s it became illegal. The person who was selling it may be arrested for something that was legal until the 90’s. (I dont really get this as I’m only 11 years old but i hope this helps.)

To sum it up you’ll get in a lot of trouble and even some legal action can be taken