Okay kiddo, so you know what a triathlon is right? It's when people run, swim, and bike all in one race. But an ultra-triathlon is like a super big, extra long version of that!
Instead of just running, swimming, and biking for normal distances, people doing an ultra-triathlon will do it for WAY longer. They might run for 50 or 100 kilometers (that's like running from your house to your cousin's house and then back again a bunch of times). They might swim for 10 or 20 kilometers (that's like swimming from one end of a really big swimming pool to the other end, over and over again). And they might bike for 200 or 300 kilometers (that's like riding your bike from your town to the next town over and then back again a bunch of times).
It's just like a normal triathlon, but it's super duper long and takes a lot of endurance and strength to finish! People who do ultra-triathlons are very determined and work really hard to prepare for them. But when they finish, they feel really proud and accomplished of what they did!