It's a phrase used to summon good luck. In the US, we say "knock on wood." It's an old superstition, some believe it refers to a time when folks believed spirits lived deep inside trees and to touch or knock on the bark of the tree would summon up the assistance of the spirits.
Anyway, the modern common usage is when you're hoping for a positive outcome: "I think I did well on all my exams, touch wood...." or "Knock on wood, I finally figured out how to program my VCR."
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