You are tied up, manacled in place, or tightly held by another creature, or otherwise physically prevented from moving in any meaningful fashion.
You cannot move from your location by any means except a Teleport effect or the thing you are Restrained by moving you.
You take Disadvantage to non-Curse attack rolls and Reflex and Defense saves.
You must make a DC 15 Flat Check when you cast spells or metamagics that have a Somatic component, wasting the spell and any Mana Flux used on it if you Fail.