Okay kiddo, have you ever hiked up a really big mountain? Imagine there are some very big mountains in the world that are divided into three parts, or what we call "summits". The first summit is the lowest and easiest part to hike up, the second summit is higher and a bit harder, and the third summit is the highest and hardest part to climb up.
Now, out of all these big mountains, there are seven that are known as the "Seven Third Summits". These mountains are the third highest peak of their respective continents, and they are really tall! For example, one of them is called Mount McKinley in North America, and it is 20,310 feet tall - that's like 48 giant playground slides stacked on top of each other!
To climb up these Seven Third Summits, people need to be very skilled and experienced at mountain climbing. They also need special gear to keep themselves safe, like helmets and ropes. It's a very challenging and exciting adventure to climb all Seven Third Summits, and only a few brave people have accomplished it!