Question: "What is a cardinal in the Roman Catholic Church?"
Answer: Cardinals in the Roman Catholic Church are the most senior officials under the pope and are usually ordained bishops. They are appointed by the pope as members of the College of Cardinals.
Cardinals attend meetings of the college and are available to the pope as needed. They are his chief counselors and contribute to governance of the Roman Catholic Church. They also have various responsibilities within their committees (called congregations). For example, one cardinal oversees all the bishops around the world, and another oversees Catholic education, and so on. Other cardinals function more as archbishops, overseers of archdioceses in a particular state or region.
Cardinals also elect the pope when that position becomes vacant and have done so since 1059. Only 120 cardinals can vote in the papal selection.
The Bible does not mention cardinals or the office in general. The Roman Catholic Church does not claim that the office of cardinal is biblical. Instead, it is part of the Canon Law of the Church: "The cardinals of the Holy Roman Church constitute the senate of the Roman Pontiff and aid him as his chief counselors and collaborators in the government of the Church” (CIC 230).
The Bible describes the governing structure for the church. First, Christ is the head, and He is the supreme authority (Ephesians 1:22; 4:15; Colossians 1:18). Second, the local church is to be free from external control or hierarchy of individuals (Titus 1:5). Third, elders and deacons are to serve as the two main offices of spiritual leadership (Acts 14:23; 1 Timothy 3:8–13). The term cardinal is not mentioned in Scripture and is inconsistent with the church leadership that is described in the Bible.