You have a job and you (and probably a few other people), depend on you and the income from that job. Disability Insurance provides protection for that income. If you are injured or have a disease that keeps you from working, disability insurance will pay money to make up for some or maybe even all of the income that you have lost.
Short term: Your policy may provide that you receive money until you are able to go back to work, or for a set number of weeks, whichever comes first. Short term coverage may be for 6 weeks or 6 months, depending on the policy. The amount of the payment is usually a percentage of your income up to a maximum amount.
Long Term coverage: If your injury is serious and you will be off work for a year or more then your policy may provide coverage first under short term provisons and then, if you have the coverage, long term insurance will provide coverage for your lost income during the disability.
Social Security: If you have contributed to social security you may be eligible for disability payments from social security. (If you have worked only for the government or a school system, you may not have paid into Social security but may be entitled to disabity plans for STRS, PERS, or some similar plan) The requirements for receiving social security disability benefits are more strict than what may be required under a disability policy that you purchase or may have as abenefit from your employer. Private disability coverage usually covers you if you are disable from doing your current type of work, Social securtiy disabity only kicks in if you are determined to be so impaired that you can not engage in any type of gainful employment.