A dielectric (or dielectric material) is an electrical insulator that can be polarized by an applied electric field.
When a voltage is applied at both sides of a dielectric, electric charges inside that dielectric will re-position themselves to in the direction of the applied electric field.
Said simply, the molecules will have their “-” side facing the “+” of the electric field and their “+” side facing the minus.
If such a dielectric is placed between the 2 metal pieces of a capacitor, then it will increasing the overall capacitance.
Here’s a good Youtube video explaining what happens:
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