How does the antagonist relate to the protagonist?
Therefore the antagonist is the character that in some way has conflict with the protagonist. The antagonist will make life difficult for the protagonist, maybe on purpose or by accident. The antagonist is often referred to as the baddie but an antagonist is not exclusively the baddie, they just have to oppose the protagonist in some way.
Though the antagonist does not have to be a single character, they can also be a group of characters such as an evil force, or race of people in conflict with the protagonist.
How important is an antagonist to a story?
All stories need both a protagonist and it's corresponding antagonist as all plots require conflict and its subsequent resolution. And what better source of conflict than from other people, it's something everyone has been through and can relate to.