Social security is basically a welfare/retirement program in the US. US workers are compelled to pay into social security by the government. The money is taken out of their paycheck. Workers pay 7.5% of their wages into social security and the employer pays an additional 7.5%. In all this means approximately 15% of the money that would be going to an employee goes into the social security system.
Social security also includes forms of disability and survivors insurance.
There is concern that the social security system will run out of money as more individuals retire and particularly as the life expectancy goes up. The short term solutions to these issues has been to raise the retirement age.