Article III, Section 1

The judicial power of the United States, shall be vested in one Supreme Court, and in such inferior courts as the Congress may from time to time ordain and establish. The judges, both of the Supreme and inferior courts, shall hold their offices during good behaviour, and shall, at stated times, receive for their services, a compensation, which shall not be diminished during their continuance in office.

The "judicial power" spoken of is the ability to hear and decide cases, controversies between individuals and the government, and between private parties. The Constitution only speaks of the Supreme Court, allowing Congress to create and organize lower court levels (Article I, Section 8, Clause 9) and giving them the ability to set the size of the Supreme Court. The "good behavior" this section mentions effectively means "for life," that judges retain their judgeship for as long as they want. The compensation or "salary" they receive may not be altered during their tenures. This affirms a separation of powers since Congress has the power to set the "compensations" and could conceivably influence a judge's decision if it were not for the last sentence of this section.