When you garble something, you warp or distort it, making it hard to understand. Talking with marbles in your mouth is one sure way of garbling your speech.
When garble first emerged on the scene in the early 15th century, it meant "to sift" or "sort through." So imagine a sentence so scrambled that you have to sort through each word, trying to figure out what everything means. Garbling can happen by accident, like when your radio signal is bad and the songs get all distorted. But spies often jumble up their secret messages on purpose to protect them from prying eyes and ears.