Scrying is a rare and difficult form of magic where a person can see and hear things through magic. It can be learned academically (such as the case with Niko) or it can be ambient. Like other ambient magics if it is undiscovered it is dangerous, but instead a regular lack of control it makes a person appear to be crazy.
Besides the magic there are also other things needed for scrying, normally some clear surface. People can scry with mirrors or clear metals, on liquids or glass. The rarest form of scrying is seing things on the winds. Wind scryers are very rare. Trisana Chandler is able to do it, although she has only encountered difficulties with other people when they learned about it. Most people trying to scry the winds go mad. Zhegorz Fiavrus was mad because his magic went unnoticed for too long.
Scrying seems to be an ability which can be learned by most mages, although they don't all have the same success with it. Daja Kisubo manages to make a mirror to scry for Nia's and Jory's powers, although she didn't have much success with scrying when Niko taught all of them.