All this data is potentially out of date, and should be taken with a truckload of salt
Astronomical Units (AU) are the unit used to measure the length between Stargates and other celestial objects in a system. 1 AU is approximately 149.598.000 kilometers. It is based on the mean distance between the Earth and its sun, Sol.
All ships also have a maximum warp-speed, which is how many AU it can travel per second, though in some smaller systems, ships will lack the room to accelerate to this maximum speed. Once a pilot's ship flies to within 0.1 AU of a celestial object, the ship's HUD will display the distance to the object in kilometers.