A cotenant is one of two or more people who signs the lease to a rental property. It's different than you paying the rent and having a roommate pay you back for their portion of the rent.
A cotenant signs a lease and is responsible for directly paying the rent and upholding other responsibilities as specified in the rental agreement.
Having a cotenant's signature on the lease is better than just signing the lease yourself and hoping the other person pays you their portion of the rent each month. Because the cotenant signed, they're responsible to pay rent directly to the landlord just as you are. If they don't pay, you're not responsible for picking up the slack.
Having a cotenant also means you'll split the utilities and have someone else to help around the common areas of the apartment.
On the other hand, there's no way you can guarantee that they'll help you. You're sharing most areas of the rental property and have less privacy than if you were living alone. You also might be blamed for the damages they cause while you're living together.
While living with a cotenant isn't always ideal, it can help keep costs lower and it's more secure to have someone else with their name on the lease rather than signing the lease yourself and expecting your roommate to pitch in.