The White House Chief of Staff is a critical member of the Executive Office of the President. The White House Chief of Staff is specifically considered to be the most powerful member of the Executive Office underneath the President, as the White House Chief of Staff acts as the President's primary aide and adviser on many different matters.
The role of the White House Chief of Staff is not set in stone, however, as the exact duties and functions which the White House Chief of Staff may perform will likely vary from White House administration to White House administration. In general, however, the White House Chief of Staff is responsible for a number of duties such as managing much of the rest of the Executive Office of the President, and managing important segments of the President's time.
The White House Chief of Staff will likely be responsible for selecting some of the most important members of the Executive Office of the President on behalf of the President himself, as the White House Chief of Staff will in many ways work with such individuals on a more regular basis than might the President.
Furthermore, the White House Chief of Staff will likely determine exactly how the infrastructure of the White House staff will function for the administration in question, and the White House Chief of Staff will likely determine who gets access to the Oval Office for speaking with the President. The White House Chief of Staff will also often conduct negotiations with Congress on the behalf of the President.