A traveler's cheque is like a special kind of paper money that you can use instead of regular cash when you're on a trip to another country. They're special because they have some extra features that make them safer for you to carry around with you.
Here's how it works: you go to a bank or another place that offers traveler's cheques and you give them some of your money (like maybe $100 or $500). In exchange, they give you a bunch of traveler's cheques with different amounts on them, like a $20 one, a $50 one, and so on. You write your name on each cheque and keep them in a safe place, like your wallet or your suitcase.
When you're traveling and you need to buy something or pay for something, you can take out one of your traveler's cheques and give it to the person you're buying from. They'll look at it and make sure it's real (because some bad guys might try to make fake ones), and then they'll give you the right amount of local money in exchange. For example, if you gave them a $50 traveler's cheque and you were in France, they might give you 40 euros back.
The reason that traveler's cheques are safer than regular cash is that if you lose them or they get stolen, you can call the bank or the company that issued them and they will replace them for you. They'll ask you some questions to make sure you're the real owner of the cheques, but then they'll send you new ones so you won't be out all that money.
Traveler's cheques used to be really popular a long time ago, but now most people use credit cards or debit cards instead. But some people still like them because they're a little bit more old-fashioned and nostalgic, and they feel like they add to the adventure of traveling. Plus, they're still a good backup option in case something happens to your other cards.