As pointed out in the sister trope Streaming Stars, there's a problem portraying motion in space. Moving in sublight speed, even if you're going insanely fast by Earth standards, you're going to look like you're standing still judging by the distant starfield and even relatively close planets. So you need a way to convey motion.
Hence, Space Flecks.
As for what they are... well, they're flecks. Of stuff. They tend to be distributed fairly evenly across the screen, too, and always in imitation of a starfield, for some reason. They all but vanish when you're standing still, usually blending seamlessly into the pinpoints of actual background stars.
You could almost say this is the video game version of Streaming Stars, given that Space Flecks are far more common in video games than on TV, and conversely, Streaming Stars aren't seen so often in games (which make use of static skyboxes) except where FTL is involved. There's additional fun to be had when they appear on planetary levels, too, where the Fridge Logic of their existence kicks just that little bit harder.
- The ancient 8-bit Atari game Star Raiders had this and might have been the Trope Codifier.
- Star Wars: Starfighter
- The Colony Wars series
- In the X-Universe, flecks of dust will appear when your ship is in motion. Some sectors are accompanied by region-themed clumps of other stuff, such as red nebula gas.
- No Gravity to a slightly painful degree, where they can also be found on planet-bound levels.
- EVE Online
- The upcoming Black Prophecy uses black chunks of space debris and a set of Speed Stripes encircling the center of the screen.
- Wing Commander had this in at least the first two games.
- FreeSpace calls it "motion debris". The upgraded FS2_Open engine has the option to disable it for the sake of realism.
- Oolite, though in-universe it's suggested that they're actually a function of the HUD to provide a visual reference speed and direction of travel.
- Tachyon: The Fringe has this, but it's justifiable in that almost all operations are within heavily colonised/industrial areas, and therefore there WOULD be a higher density of randomised crap floating around and you can disable it in the settings.
- The Independence War series averts this. Motion lines are generated by the ship's onboard computer and projected onto the HUD to provide a visual reference instead. You can turn your HUD off, but doing so makes it impossible to gauge your ship's speed and direction of motion.
- All three Escape Velocity games do this.
- Naev even tells you in the tutorial that this can be used to gauge your speed. (That was before they changed the star field to a visible background and gave you a speedometer on the HUD but you can still see stars slightly.)
- A rare but iconic television version is popular in Star Trek: The Next Generation. Whenever a scene shows a window or exterior view while the ship is moving at sublight speeds, the star field moves slowly but very noticeably in the background. This is made possible by a large roll of black fabric marked with a random collection of pinpricked holes, which is then spooled very slowly and heavily backlit while filming, giving the audience a field of hundreds of stars while providing the illusion of the Enterprise in motion. Obviously, then can lead to continuity errors, when the star field moves in a direction opposite the ship's course at the time, or they do not show a planet the ship is orbiting, and so on.