A tribunal is like a court. If you've done something wrong and you get caught it's likely you'll be brought before a tribunal, although not all tribunals are there to determine guilt.
Say, for example, you'd like to join a secret society. That society might convene a tribunal to determine whether you're worthy of joining. One way to remember tribunal is to focus on the tri at the beginning of the word. A tribunal is there to "try" a case. A tribunal is a group of people who oversee a trial. Your mom and dad could be a tribunal. You could also think of it as the group who oversees their tribe, which is, in fact, where the word comes from.