Dielectric materials:

Just as the certain solids can be placed within the coil to increase the inductance, materials exist which can be sandwiched in between plates of a capacitor to increase capacitance. The substance between the plates is known as dielectric of the capacitor.

Air works well as a dielectric, it has no loss. But it is difficult to get capacitance using air as the dielectric. Some solid material is generally employed as the dielectric for most fixed capacitors, which means that for types manufactured to have a constant, value of capacitance.

Dielectric materials conduct electric fields well, but they are not the good conductors of electric currents. Actually the materials are generally known as good insulators.

Solid dielectrics increase capacitance for the given surface area and spacing of the plates. Solid dielectrics allow the plates to be rolled up, squashed, and placed close together. Both of these act together to increase the capacitance per unit volume, allowing the reasonable capacitances to be there in a small volume.

Figure--   Foil sheets can be rolled up with dielectric material sandwiched in between.