Nuclear fusion; the driving force behind all stars. It is the force that makes everything sentient life, or life in general, takes for granted; light, planets, life itself, all originated from this fundamental principle of existence, so important, yet so simple. Two atoms, under the ever-present influence of gravity, fuse and create a heavier atom, releasing energy due to the fact than not all of the original atoms are preserved; the leftover is converted into energy. This process is usually energy efficient (read: energy is gained) under any atom lighter than iron; anything heavier requires impute of energy (this is a major factor in supernovas and generally any related star death).
The most important application of this phenomenon is in fusion reactors. Using hydrogen, or any other suitable fuel source (helium 3, deuterium, tritium) in large magnetic confinement units, using electromagnetism to fuse them instead of gravity. The largest problem with the technology is plasma containment issues, an issue that most advance civilizations can solve rater early by implementing electromagnetic technology, and later by using laser-based techniques.
Yet another, even more important implementation of fusion is space travel. By building a spacecraft around an advance fusion drive, the vehicle, given enough fuel, can become one of the most advance vehicle invented. Not just by using the power generated to sustain itself for almost indefinite periods of time, but also for potential use as the very first spacecraft with a reasonable travel time between planets, using the plasma generated by the reactor instead of conventional chemical rockets. A fusion torch. This "engine" can propel the craft to a fraction of the speed of light, reducing traveling time for weeks or even days.