Zoning means the act of playing more defensively. Lets take Kano for example, he has a knife throw and upball that can counter making him more of a defensive character. If you start throwing knives at your opponent, their first reaction would be to jump. So they keep hopping over your knives but when they in range of your upball you use it knocking them away from you. The upball will then send them flying back and the process will repeat unless they have a teleport character like Raiden. This scenario might/might not happen depending on how good your opponent is and they might dash block instead which is a bit more complicated to explain.
Zoning is controlling your space: from projectiles to other long range attacks, its keeping space between you and your opponent and keeping them from getting to close.