The net magnetic flux through any closed surface is always zero — isolated magnetic poles (monopoles) do not exist.
Every line leaving N returns to S — in through one side of any surface, out the other.
Φ(B) = 0 (∮ B · dA = 0)
Φ(B)
net magnetic flux through a closed surface (webers)
B
magnetic field (teslas)
In plain English: Magnetic field lines never start or end — they always close into loops. Snap a magnet in half and you get two smaller magnets, never a lone north pole. Whatever flux enters a closed surface must leave it.
Where you'll meet it: Why every field-line diagram you draw must close on itself, why transformer cores are designed as closed magnetic loops, and one of Maxwell's four equations.