"Subspace sandbar" was a term coined by the crew of the USS Voyager to describe a region of space which intersected a transdimensional realm inhabited by photonic lifeforms. In this region, the area between space and subspace had become unstable, allowing a subspace layer to extrude into normal space.
Voyager encountered this phenomenon in 2375, running aground of the subspace layer and subsequently experiencing a disruption of the ship's control systems due to gravimetric forces disrupting the ship's power flow. This had the effect of denying the crew access to their computer core, tactical systems, holodecks, and all but six replicators. Attempts to free Voyager from the sandbar by realigning the warp field, reversing hull polarity, and powering down the warp core and using minimal thrusters proved fruitless. It was soon discovered by the crew that the photonic lifeforms which inhabited the transdimensional realm had mistaken the holodeck character of Doctor Chaotica for an actual entity, due to the hologram's photonic nature, and had become engaged in an armed conflict. Making contact with the lifeforms, the Voyager crew managed to negotiate a cease fire long enough to defeat Chaotica and end the battle. With Chaotica's defeat, the photonic lifeforms returned to their realm, closing the subspace distortions they were using to access normal space, subsequently allowing Voyager to cross the subspace barrier and escape. (VOY: "Bride of Chaotica!")