Once you're in space, whether by launching a rocket or departing a space station, you'll need some way to propel your spacecraft toward its destination. That may require you to carry an on-board fuel source. Ideally, you'll have an efficient system so that you don't have to dedicate too much space to carry fuel. One potential solution is fusion.
Fusion is the method by which the sun generates energy. Under intense pressure and heat, hydrogen atoms smash into each other and form helium. Hydrogen has a single proton and helium has two of them. During this process in which two hydrogen atoms fuse together there's a release of neutrons and energy.
But there's a big problem -- we haven't figured out how to use fusion to generate power in a reliable and sustainable way. The process requires incredible amounts of heat and pressure. Just generating the conditions necessary for fusion can require a great deal of energy all on its own. The goal is to reach a point where we can initiate fusion and keep the process going while we harvest energy. We're not there yet.
If we ever do get there, fusion may be a good choice for powering spacecraft. We could harvest a great deal of energy from a comparatively tiny amount of fuel. Fusion could generate the power necessary to operate thrusters to allow for in-flight adjustments as we fly our way to the next planet over. But whether fusion is a practical option remains to be seen.