When you're "disappointed", it means that you're a little sad because something wasn't as good as you expected it to be. You can be "disappointed with" a movie, a sports team's performance, and so on:
Betty was kind of disappointed with how her cupcakes turned out.
But when you say that you're "disappointed in" a person, it means that someone who you used to have a high opinion of did something bad. So now you can't respect that person as much.
The phrase "disappointed in ___" is usually used when you're talking about your children, your employees, or someone else with lower status. If you directly tell someone that you're disappointed, it's a pretty serious statement and makes it sound like you have authority over that person:
Son, I'm disappointed in you.
You might tell someone this if you found out that they did something immoral like lying, cheating on their partner, or stealing something.