(covers information from several alternate timelines)
A subspace bubble was an encompassing energy field which could be used to protect objects or people from various environmental effects.
In 2371, an exploding microscopic singularity passing through the Sol system created a subspace bubble around the USS Defiant, isolating it from changes in the timeline inadvertently effected by three crewmembers who had traveled back in time to the year 2024. This allowed the Defiant crew to observe an alternate timeline in which Starfleet apparently did not exist and Alpha Centauri was under the control of the Romulan Star Empire. The timeline was later restored when the crew was able to ensure that events in the past unfolded as they did in the original timeline. (DS9: "Past Tense, Part I", "Past Tense, Part II")
A subspace bubble could be formed around a person. This variation was used by Seska on a Kazon raider to bypass a force field, in order to retrieve a Federation replicator that she had traded in turn for a Kazon-Federation alliance. (VOY: "State of Flux")