The retirement earnings test is how Social Security determines how much to withhold from your benefits if you claim benefits below normal retirement age and are still working. Typically, the withholding is larger in the years before you reach normal retirement age, gets smaller as you age, and is tied to the cost of living.
Also, any benefits withheld while you continue to work are not lost. Once you reach full retirement, your monthly benefit will be increased to account for the months in which benefits were withheld.
Want to estimate what your specific withholding might be? Social Security offers a retirement earnings test calculator here.