An interdenominational marriage happens when two people who believe in different things get married. Let's pretend you like chocolate ice cream and your friend likes vanilla ice cream. You both really enjoy ice cream, but you have different opinions on which flavor is the best. This is kind of like how different churches have different ideas about religion. Someone who goes to a Baptist church might believe one thing while someone who goes to a Methodist church believes something else.
When two people from different churches decide to get married, they might have different opinions about religion. For example, one person might want to get baptized while the other doesn't. Or one person might think that taking communion is really important while the other person doesn't think it matters as much.
This can be a bit tricky because both people want to feel like their beliefs are being respected. But just like how you and your friend can both enjoy ice cream together, it's possible for people from different churches to have a happy marriage too. They just need to have open and honest communication, listen to each other's opinions, and figure out how to make compromises so that both people are happy.