A boat lift is like a big sippy cup for boats. When you want to drink from your sippy cup, you use your hands to lift it up to your mouth. The boat lift does the same thing, but for boats.
The boat is put into the boat lift, kind of like when you put your sippy cup into a holder. Then, the boat lift uses strong motors and pulleys to lift the boat up out of the water like a magic spell, so it doesn't get wet or damaged. This makes it perfect for boats that need fixing or cleaning, or for boats that are too big or heavy to be moved by people alone.
Most boat lifts are shaped like a giant fork, with two parts on either side that cradle the boat, and a big metal beam that lifts the boat into the air, like a forklift lifting a box. The boat lift is kept stable and safe by being bolted down into the ground and secured with poles that stop it from toppling over.
So remember, just like how you use your sippy cup to take a drink, a boat lift is used to lift boats up and out of the water using lots of strong machines and special parts, so they don't get wet, dirty, or damaged.