Think of the American Association of State Highway and Transportation Officials (AASHTO) as a big group of people who work together to make sure our streets, roads and highways are safe and easy to travel on.
You know when you go on a road trip with your family and you see signs that say things like "55 MPH" or "Exit 12"? Well, the people who work at AASHTO make those signs and decide where they should go.
They also make sure that the roads are built to be strong enough to hold up all the cars and trucks that drive on them. They come up with rules for how wide the lanes should be, how tall the overpasses need to be, and how strong the guardrails should be to keep you safe.
When there's a problem with a road, like a big pothole or a bridge that's falling apart, the people at AASHTO talk with each other and try to figure out the best way to fix it. They might suggest putting more gravel on the road, fixing the pothole with some special material, or even building a brand new bridge to replace the old one.
So basically, AASHTO is a group of grown-ups who try and make our roads better and safer for all of us to drive, ride or walk on.