Things sink if they're denser than the water or whatever liquid they're in.

Density is the weight [or mass] of something per unit volume. Water has a density of 1 gram per milliliter.*

A marble, for example, is heavier than the amount of water that's the same size as the marble, so the marble sinks.

* The density of a substance is its mass per unit volume.

Some things that sink in water will float in salt water, because salt water, with salt + water in it, is denser than water.