A pulley is a machine that is used to lift objects.
More pulley facts
Window washers use pulleys to wash to Windows on tall buildings so when they want to move up they pull on one of the strings. So when the washers pull on one of the strings the other string goes down while the other one goes up.
Again more pulley facts
People use pulleys for many reasons like window washers or construction workers or engineers. Most of time people use pulleys for lifting very heavy stuff. Back in the day people could use them because they didn't need electricity, all you need is a rope and a grooved wheel and then you can make one