The minimum gap between copper of different nets; set by the fab (0.1 to 0.15 mm on our boards).
How it looks
Real geometry: CORE3588 copper, drawn from our board file (2D plan view)
Top copper, 8 x 6 mm near the right-hand power supply. The mid-grey sheet is the ground pour; light grey is copper of other nets (and the ground pads). Each black outline is clearance kept on purpose.
1A signal via inside the ground copper. The black ring around it is the clearance: no ground copper may come closer than 0.2 mm.
2The same black gap runs around every grey pad and track of another net.
Real image: CORE3588, rendered by KiCad from our board file
The same patch as the fab would make it (KiCad 3D render). Under the green mask the copper looks olive; the thin darker-green outlines are the clearance gaps.
Every net class carries its own clearance; on CORE3588 all classes use 0.2 mm, a little more than the 0.1 to 0.15 mm the fab could make, which leaves margin. A short circuit is what clearance prevents.