According to the Kinetic Molecular Theory, the kinetic energy of the particles of a solid is low enough as for the intermolecular forces to be enough to keep the atoms of a substance roughly at the same place from each other. On the other hand, the kinetic energy of the particles of a liquid is high enough as to overcome the intermolecular forces but not as high as to free the particles completely. Then, the molecules of a liquid can move away from each other, and they assume the shape of a container into which they are poured provided that there is a gravitational force pulling the particles of the liquid toward one side of the container.
To summarize:
The kinetic energy of the particles of a liquid is enough to allow them to move with respect to each other, whereas the kinetic energy of the particles of a solid is not enough to overcome the intermolecular forces, so they remain at the same place with respect to each other.
