Have you ever sat and watched what happens when a beach ball gets hit or bounces off of the ground? Or seen what happens when someone places a plate of gelatin on a table? Or observed how a person's hair moves when they shake their head? When these things move and collide with other objects, they have a bit of internal jiggle that can be difficult to reproduce correctly with regular animation tools. This jiggling is the basis for what is referred to as soft body dynamics.
You can simulate soft body dynamics in Blender from Physics Properties. Left-click the Soft Body button, and a Soft Body panel appears. In that panel, you can make adjustments and tweak the behavior of your soft body simulation.
Like with particle systems, adding soft body dynamics to an object from Physics Properties also adds a Soft Body modifier to your object. You can verify this addition by looking in Modifiers Properties.
What follows is a simple step-by-step process for creating a simple soft body simulation with the default cube object:
You want to give the cube some height to fall from. It doesn't have to be very high; 3 to 5 units should be enough.