This is my attempt to explain visual odometry (<-- the only words that are not in the ten hundred most used words!) using only the ten hundred most used words. I made it using the following place.
When a space car moves, it does not usually move exactly as far as it thinks. Sometimes it moves a little more, and more often it moves rather less. Sometimes it is stuck and does not move at all, even though it thinks it moved. Since no person is there to watch it, this is a problem for the space car: the space car might go off course and get hurt, or it might get stuck for always, never able to move again.
But we made up a way for the space car to figure out how far it really moved, so it can stay safe. It does this using pictures.
Before it moves, the space car takes a picture. Then the space car moves a little, and then it takes a second picture. It finds the same points in the two pictures. By seeing how far those points seem to have moved, the space car is able to figure out how far it really moved. Then it fixes its idea of how far it moved, so now it knows where it really is.
Then it does that again, and again, and again, as many times as it needs to, until it gets where it wanted to go (or it runs out of time first, but that does not happen often). If it notices it's stuck, it stops moving until the next day, when one of us humans can help it.
Taking all these pictures is much slower than not taking them, so we don't take them all the time, only when we need to. But it works very well. It has helped our space cars stay very safe, so it is important to us. Space cars are a lot of money, and there are not many of them! And we fall in love with them, too, so they are also important to us for that reason.
(Writing using only the ten hundred most used words is hard, but it is good to do. What do you think of my try? It makes what I write sound like a book for children sometimes, but that is okay: it makes sure I understand hard ideas well enough to explain them to others, and that is good for someone who likes sharing with as many people as possible.)