The Sun generates its energy when two protons (hydrogen nuclei) collide and fuse together to form helium. As the two protons approach towards one another and finally merge, their individual masses decrease; the "lost mass" \(Δm\) is converted into energy according to \(E=(Δm)c^2\). Fusing together hydrogen nuclei is not yet practical due to the extremely high temperatures required to make the nuclear reaction happen. But it seems likely that some time during the 21st century, we'll be able to fuse together two heavy isotopes of hydrogen called dueterium (which is an atom whose nucleus contains one proton and one nuetron) and tritium (an atom whose nucleus contains one proton and two nuetrons); it also seems very likely that, within the 21st century, we'll be able to fuse tritium and an isotope of helium called Helium-3 (or \(H_3\) for short). These resoures are fairly ubiquitous in our solar system. Dueterium and tritium are constituent of water (\(H_20\)) which can be found frozen (and in large quantities) on Mars, in both of our solar system's asteroid belts, and in the Ooort Cloud. Helium-3, despite its rarity on the Earth, is also pretty ubiquitous in the solar system. Over the course of billions of years, solar winds from the Sun has deposited over one million tons of \(H_3\) into the Moon's regolith and also many tons of the stuff into asteroids within the asteroid belt between Mars and Jupiter. It is concievable that future human space explorers will construct enormous arrays of solar panels on the surfaces of those asteroids; the solar energy generated would be sufficient to power at least some of the domed cities thereon. But to actually move those little rocky worlds to elsewhere in the solar system (for example, into Earth's orbit) or to another solar system entirely, solar energy alone would be insufficient. But nuclear fusion would do the job. Factories could be built along the asteroidal surfaces and used to extract water from the immense repositories of frozen ice, and all of the \(H_3\)—blasted into the surface by solar winds—could be extracted as well. The entire asteroid would be its own cosmic gas station and its own source of energy; those raw ingrediants could be used to sustain nuclear fusion for many eons allowing any voragers on board those asteroids to traverse the black sea of empty space and to power their civilizations for many eons.
But during the midst of the second World War, a far more imaginative and efficient way of generating power and moving asteroids using nuclear fusion was developed. It was during this time that the use of matter—anti-matter engines was first suggested. What is anti-matter?, one might ask. Anti-matter is essentially identical to matter except that the charges of the particles comprising the stuff are reversed. For example, an anti-hydrogen atom would be the same as regular hydrogen except that the proton becomes negatively charged and the electron becomes positively charged—the magnitudes of the charges in anti-matter and regular matter are the same, but their signs reversed. Anti-matter might sound so exotic as to be confined to the realms of science fiction. But this is not the case. We have created very minute quantities of the stuff in particle accellerators. Throughout the 21st century, the techniques used to manufacture anti-matter will improve and it seems very likely that we'll use anti-matter (produced by particle acellerators) to power future matter—anti-matter engines. But how is energy produced by such an engine? When atomic nuclei (both composed of ordinary matter) collide, there masses change by a small amount \(Δm\). But when matter and anti-matter collide and fuse together, they annalihate—all of their mass is converted into energy. This makes matter—anti-matter engines a far more efficient power source than the aforementioned nuclear fusion involving only ordinary matter. Both methods will still, nonetheless, be used by future, space fairing, human explorers as they traverse the Milky Way galaxy.
Sources: Kurzgesagt – In a Nutshell