What do these terms mean?

Immediate relative - Spouse, widow(er) and unmarried children under the age of 21 of an American citizen.  A parent is an immediate relative if the American citizen is 21 years of age or older.  There are no numerical limits to immigration of immediate relatives.

Preference Immigration – A system for determining which and when people can immigrate to the United States within the limits of immigration set by Congress.  In family immigration, preference is based on the status of the petitioner (American citizen or lawful permanent resident) and his/her relationship to the applicant.  In employment immigration, it is based on the qualifications of the applicant and labor needs in the United States.  All preference category immigration (excludes immediate relatives) are subject to numerical limits.

Numerical limitations – Some form of numerical limitation has been imposed on immigration into the United States since 1921, although certain classes of immigrants have traditionally been able to obtain visas outside those limitations. The Immigration Act of 1990 (Pub. L. 101−649) modified this concept by establishing an overall limit within which immediate relatives (IR's) and certain special immigrants would continue exempt from the requirement of an available visa number, but the IR total would be deducted from the overall ceiling for the calculation of the family-sponsored preference ceiling during the following year. You can find more information here.

Priority date – The priority date decides a person's turn to apply for an immigrant visa. In family immigration the priority date is the date when the petition was filed at a Department of Homeland Security (DHS), U.S. Citizenship and Immigration Services office or submitted to an Embassy or Consulate abroad.  In employment immigration the priority date may be the date the labor certification application was received by the Department of Labor (DOL).

Cut-off date – The date that determines whether a preference immigrant visa applicant can be scheduled for an immigrant visa interview in any given month.  The cut-off date is the priority date of the first applicant who could not get a visa interview for a given month.  Applicants with a priority date earlier than the cut-off date can be scheduled.  However, if your priority date is on or later than 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.