A lease is a legal agreement between you and the person (or company) from whom you rent. Leases obligate you and your landlord to particular commitments.
The most important obligation of your lease is the length of your stay. When you sign a lease, you agree to rent the apartment for a certain period of time. This means that you must pay the rent on the apartment for the entire time of your lease. Apartment leases typically last for one year.
Your lease obligation applies whether you live in the apartment or not. If you choose to leave before your lease is up, your landlord may legally collect the rent you owe for the time remaining on your lease. So, before you sign, be very sure that you can meet the terms of your rental agreement.
Why sublease? You may find good deals. If you are looking for housing only during the summer months, or during a period when the original tenant will be gone for a few months (to study abroad perhaps), the original tenant may be willing to negotiate the rent in order to keep the lease and have a place to return to after the summer or study abroad program. It's worth asking!