What it means
Mercury never moves more than about 28 degrees from the Sun, so the two meet often. Budhaditya means "sun and mercury together". The result is usually described as clear thinking, skill with words and numbers, and the ability to be heard.
Here is the catch. Mercury close to the Sun is often combust, scorched by it, and then it loses its independence. Many astrologers say a combust Mercury spoils the yoga. So this rule asks for both: the two planets together, and Mercury still clear of combustion.
Sun and Mercury share a sign in about half of all charts. Add the combustion check and about 1 in 3 remain. That is on purpose. Passing means the Sun gave Mercury authority without swallowing it.
Check yours. If it fails only on combustion, the pair is still together, but tradition reads the yoga as weaker.