Things, conditions, or phenomena that are circumstellar revolve around or exist in the region around a star. For example, some stars have circumstellar disks of gas, dust, and debris orbiting them.
In addition to circumstellar disks, there are circumstellar winds — streams of particles that flow outward from a star. Astronomers also talk about "circumstellar habitable zones," where conditions around a star can support life as we know it. The star at the center of our solar system is the Sun; of the planets orbiting the Sun, only Earth is in its circumstellar habitable zone, while Venus and Mars are on the edges of it. Circumstellar is built from circum-, "around," and stellar, "of the stars."