What phases of matter are compressible?

Gases and Liquids are compressible.

In a gas, the particles are very far away from each other, so there is a lot of space for the particles to be compressed down in to. This is very helpful when transporting gases in containers.

In a liquid, the particles have slightly overcome the forces holding them together, so they are able to slide past each other, but the space between them is small. A liquid can be compressed, but only slightly.

In a solid, the particles are locked into a rigid structure, and are vibrating in place, but are unable to move. The particles in a solid are already as close together as they are going to get, so no compression is going to happen.

The following simulation on the PhET website helps by giving a visual explanation of this concept.https://phet.colorado.edu/en/simulation/states-of-matter-basics