Most seas in the world are about 3.5% salt. The Dead Sea is almost 10 times saltier than that - around 34% salt. That much salt makes the water heavier than your body, so the water pushes you up and you float without trying. People love to lie back and read a book on the surface, just to prove they can.
It is called the 'Dead' Sea because the water is too salty for fish or plants to live in. The only things that survive there are tiny microbes. Around the edges, salt builds up into strange white crystal shapes that look like alien rocks.
The shore of the Dead Sea is the lowest place on dry land in the world. If you stood there and someone stood on top of Mount Everest, you would be more than 9 kilometres lower than they are. Your phone's altitude app would show a minus number.
The black mud around the lake is full of minerals and is famously good for the skin. People travel from all over the world to slather themselves in it and let it dry in the sun. Many soaps and creams sold in shops list 'Dead Sea minerals' on the label.