In the process of creating magnetic containment to make fusion power generations possible, humanity got significantly better at manipulating magnetic fields. In the process, they discovered how to miniaturize the process, creating the Aegis field.
These fields are sensitive enough to polarize both matter and waveforms, and are usually calibrated against a certain velocity – permitting the passage of matter and waves only below a given energy level. For a ship, this threshold is fairly high – almost any speed or energy is too high to be allowed inside, and solar panels are extended outside of the Aegis field to collect solar radiation.
In miniaturizing this technology, however, the Terran Federation has created Aegis fields for personal defense as well. These can either have their settings tuned low, permitting the passage of atmosphere, or high, allowing them to serve as a substitute for clunky environmental vacuum suits. Generally speaking, they permit the passage of an object moving no faster than six centimeters a second.
Humanity has found no real way to make fusion work without this technology, and no feasible way for large-scale space exploration without fusion, so they consider this prerequisite technology – any existing alien species would almost have to also have this technology to become an interstellar species.