Social Security is a trust set up by the Federal Government. You pay into the trust as you work and when you become a senior you get money dispensed from the trust proportional to what you paid in. During the time in between, the Trust grows due to interest and some other small investments.

Congress has been raiding the Social Security Trust since the late 70s to pay for programs in the general tax pool and not paying it back because these people campaigned on lowering taxes and found a way to keep that promise while also not reducing expenditures. Over 30 years, they pulled out so much money, that the Trust now doesn't have enough money to build interest to keep up with the amount of people currently reaching retirement age.