Question

why does the level of water increases when any solid substance is thrown in it

Answer 1

An object is always pulled down due to gravity. However, in water (rather fluid) the object also experiences an upward force called the Buoyant Force. Force of gravity depends on the density of the object while the buoyant force depends on the density of water.

If the density of the object thrown is lesser than the density of water then it will float otherwise sink.
The water level is increases only when solid thrown get sink inside the water. This will change total volume of the system and water level increases.