Freezing-point depression is the process in which adding a solute to a solvent decreases the freezing point of the solvent. When you add solid particle in a liquid it hinders the interparticle attraction of the liquid particles, that's why their freezing point decrease. More precisely it increase the entropy of the liquid, therefore the liquid must be subjected to even lower temperature so that it can attain it's solid form. Now , what happen at freezing point, the liquid and solid form of a substance must be in equilibrium , so answering to your question, The liquid solvent and solid solvent form are in equilibrium.
d. Solvation of Venic 9. During depression in the freezing point in a solution, the following are in equilibrium. a. Liquid solvent, solid solvent b. Liquid solvent, solid solute c. Liquid solute, solid solute (IIT-JEE, 2003) d. Liquid solute, solid solvent hint of a solution of 13.44 g of