The amount you pay for health care before your insurance kicks in. For example, if your deductible is $1,500, you will pay the first $1,500 in medical bills out of your own pocket. Deductibles are cumulative over the course of a calendar year – each time you pay a small bill, it counts towards your deductible. Once you've reached your deductible, your insurance will begin to pay the majority of medical expenses. Each year on January 1st, your deductible resets and you start all over.
Plans with low premiums generally have high deductibles. High deductible plans are best suited for people who are generally healthy, don't expect to go to the doctor often, and only need catastrophic coverage. Remember, with high deductible plans you should put some money aside so you're prepared to pay your deductible if necessary.
Some plans have very low (even $0) deductibles, but they also have much higher premiums. These plans are best for people who expect to have a lot of medical expenses or would rather pay-upfront for some extra peace of mind.