A Geas is a spell used to seal an oath or to force someone to do the caster's bidding. Should the target of a geas spell fail in their quest or break their oath, they will suffer dire consequences, like grievous pain and/or death.
The geas spell cannot be dispelled by any conventional means like Dispel Magic or Remove Curse. The only way to remove geas in the first game is to use the Geas Removal Scroll from Jalantha Mistmyr during the quest Marek and Lothander.