Imagine you have some toys that you want to trade with your friend. You have a toy car and a doll, while your friend has a toy truck and a stuffed animal. You and your friend agree to trade, but you need to decide how many toys you want to exchange for each other. This is called the "terms of trade."
Your friend suggests that they want both your toy car and your doll in exchange for their toy truck, but you don't think that's a fair trade. You want to keep one of your toys, so you suggest that your friend gives you their toy truck and stuffed animal for just your toy car. That's a good deal for you! You now have their toy truck and a new stuffed animal, and you only gave up one of your toys.
This is similar to what happens in international trade. Countries exchange goods and services with each other, but they need to agree on the "terms of trade." The terms of trade are the exchange ratio of one country's goods for another country's goods. For example, if the terms of trade for China and the United States are that China trades two pairs of shoes for one computer from the US, then that is the ratio they use to exchange those goods.
It is important for countries to have good terms of trade because if one country is always giving up more than they receive in trade, then they are losing out. It's like trading two toys for one toy - that's not a good deal! So, countries need to negotiate to get the best possible terms of trade for themselves. That way, they get what they need while giving up as little as possible.