A marquee is like a big tent that people use for parties or events. It's a bit like a house, but without walls - just a roof held up by poles.
Imagine a really big umbrella, but with stronger legs that can stand up by themselves and hold up a roof. People make marquees out of special fabric that's strong enough to keep out sun and rain. They also have windows and doors so you can go in and out easily.
At a party or event, people set up tables and chairs inside the marquee to have a place to eat and drink or to play games. Sometimes people also decorate the inside with lights or flowers to make it look pretty.
So a marquee is like a special tent used for parties and events to keep you dry and shaded from the sun.