There are many reasons why you could be tempted to break a lease. You might get a new job in a different city. You might find a better deal on an apartment in a better neighborhood. Before you pack up and go, you do need to know the current landlord still expects you to fulfill the current lease. If you leave without addressing that lease, it could give future landlords a negative opinion of your reliability.
At the very least, a broken lease means you'll lose your security deposit. That’s bad news, but things could get worse if your landlord shares news of your behavior with others. When you move to another rental unit, the landlord there will almost certainly ask for references from past landlords. He’s unlikely to be thrilled to learn you broke a lease. He’ll regard you as a risky bet, and may be unwilling to rent to you.
Large landlords managing multiple units may be in touch with the credit reporting bureaus. Even small landlords can report lease breakers to the bureaus. If the landlord wants, he could put a note on your credit report that says you failed to pay as agreed. That note will hurt your rental status and your overall credit rating.
Sometimes, there are mitigating circumstances that give a renter just cause to break a lease. If a landlord refuses to keep the unit in good repair, or the renter has constant problems with neighbors, the renter could be justified in leaving before the lease runs out. That could still lead to some problems if the landlord disagrees with those circumstances. The renter needs to document all the problems in writing. That means you'll need to keep copies of things like repair requests and police reports. Show that list to the next landlord if she wants evidence for why you broke a lease.
When you break a long term lease, you are ignoring a legally binding contract. The landlord can sue you in small claims court for all of the rent due for the rest of the lease term. That's the immediate problem. The long term problem is a victory by the landlord means a judgment against you on your credit report. It will be there for seven years and will certainly raise a red flag with a possible landlord in the future.