A car boot liner is like a big waterproof blanket that goes in the back of your car. It's really good at protecting your car from getting dirty or wet if you put things like muddy shoes, sports equipment, or gardening tools in the back.
Imagine if you spilled your milk on a table: your mum probably put a tablecloth over the top so the milk didn't get on the table. The car boot liner works the same way: it covers the bottom of the car boot so things you put in there don't make a mess.
The liner is shaped like the bottom of your car boot so it fits really well, and it's usually made of a strong material like rubber or plastic so it'll last a long time. When it gets dirty, you can just take it out and give it a rinse.
So, basically, a car boot liner is a big waterproof blanket that keeps your car clean and protected from messes!