Soilds retain their shape because their particles (be they atoms or molecules) are locked in position by bonds or intermolecular forces of neighboring particles. When you heat a solid so that it changes phase to liquid form, the thermal energy put in serves to excite the particles of matter enough so that they are still held together but can now overcome the forces holding them rigidly in place and slides around each other. Liquids are virtually incompressible because the particles are still held together, but they can also flow and change shape because they're not fixed to the same arrangement.
