The diagrams show the movement of particles for two substances.
The left image labeled X shows spaced out green balls with long movement lines behind them. The right image shows tightly cluster green balls with short movement lines behind them.
The substance in the left image, X, is a gas. The particles are spaced out and have long movement lines behind them, indicating that they are moving quickly and with a lot of energy. Gas particles have a lot of kinetic energy and are not held together tightly.
The substance in the right image is a solid. The particles are tightly clustered together and have shorter movement lines behind them, indicating that they are vibrating in place. Solid particles have very little kinetic energy and are held together tightly by strong forces of attraction.