Okay kiddo, an ocean liner is a very big and fancy boat that sails across the ocean. It's kind of like a hotel on water, with rooms for people to sleep in, restaurants for them to eat at, and other fun things to do on board.
But ocean liners aren't like your normal boats that you see at the lake or river. They are much bigger and can carry thousands of people at once. They are also built to be very strong and stable so they can sail through storms and big waves without tipping over.
People use ocean liners for a special kind of trip called a cruise. This means they get on the boat in one city and sail to another, visiting different places along the way. It's like a big adventure on the water!
Some people even live on ocean liners for weeks or months at a time, traveling around the world and seeing all kinds of exciting things. It's like a really long, fun vacation that never ends!
So that's what an ocean liner is, a big and fancy boat that people use to travel across the ocean and have lots of fun while doing it!