A kilometre is a way to measure how far something is. It's like how you use your hand to measure how tall you are. A kilometre is a really big way to measure distance, sort of like how a grown-up might use a really big ruler to measure a room.
When you go for a walk, you might go a few hundred metres, which is like taking a few big steps. But if you go on a really long trip, you might go thousands of kilometres, which is like taking a really, really, really big step.
Some countries use kilometres instead of miles to measure how far things are. It's sort of like how some people use different words for the same thing - like how some people call a fizzy drink "soda" and some people call it "pop". Kilometres and miles are both ways to measure distance, but they're just different words for the same thing.