There are numerical limits on the number of immigrant visas that can be granted to aliens from any one foreign country. The limit is based on place of birth, not citizenship. Because of the numerical limits, this means there is a waiting time before the immigrant visa can be granted. The terms current/non-current refer to the priority date of a petition in preference immigrant visa cases in relationship to the immigrant cut-off date. If your priority date is before than the cut-off date according to the monthly Visa Bulletin, your case is current. This means your immigrant visa case can now be processed. However, if your priority date is later/comes after the cut-off date, you will need to wait longer, until your priority date is reached (becomes current). To find out whether a preference case is current, see the Visa Bulletin or telephone (202) 663 1541.
Immediate relative immigrant visa cases do not have country numerical limits, with waiting times as a result of the country limits. The terms priority date, cut-off date and current/non-current does not apply for immediate relative cases.
Source: U.S. Department of State.