A person who is of Cabinet rank holds a ministerial position in a government. They sit in Cabinet and form the highest body of leadership in a government.
In the UK, tradition holds that the top three Ministers after the Prime Minister are the Chancellor, Foreign Secretary and Home Secretary. In that order these 4 positions are referred to as the Great Offices of State, the most prestigious positions in government. However the high prestige of these positions has not always run with Cabinet rank. In the modern political era Cabinet rank is often a gift of the Prime Minister who releases a list detailing the seniority of all Cabinet ministers. Sometimes positions that one would not expect to be as high ranking as say, the post of Foreign Secretary can in fact be placed higher than a Great Office of State. This is best displayed during the premiership of Gordon Brown when after a reshuffle of his Cabinet Peter Mandelson, the Business Secretary was ranked the 3rd most senior minister out of 23 and outranked all Great Offices of State except the Prime Minister himself. 
Just as traditionally lower Cabinet jobs can be placed high so can traditionally higher positions be placed low in the rankings such as when Bob Ainsworth (then Secretary of State for Defence, a very important job) was ranked the 3rd lowest ranking minister in Gordon Brown's Cabinet. 
After the UK General Election of 2010 Cabinet rankings have gone back to a more traditional pattern, with the highest ranking ministers being those holding the Great Offices of State and holding quite important posts, for example the Secretary of State for Defence and the Secretary of State for Work and Pensions. 
United States of America
In the United States the ranking of members of the Cabinet forms a part of the presidential line of succession. Typically more senior members sit closer to the President when seated in the Cabinet room.