Your heart, lungs, and belly organs are always moving. A slippery lining on the inside of your body walls helps them slide without rubbing.
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Your chest and belly are like rooms. Parietal mesothelial cells line the inside walls of these rooms, like a very thin wallpaper.
The cells are flat, a bit like tiles. They sit in a single layer, and their tops are covered in tiny finger-like bumps.
Every time you breathe, your lungs slide along your chest wall. Your heart beats and your gut squeezes too.
Mesothelial cells make a thin, slippery fluid. It lets organs slide along the walls without rubbing or sticking.
Rub your dry palms together. Now add a drop of water and rub again. The water makes them slide more easily, just like the fluid these cells make.
There is only a tiny bit of this fluid, just a thin wet film, but it is enough to keep everything sliding smoothly.