When combined, “双喜临门” literally means “a pair of happinesses arrives at the door.” This creates a vivid image of good fortune physically visiting one's home, blessing the entire family. This idiom is the linguistic counterpart to the famous 囍 (shuāngxǐ) symbol, which is a ligature of two 喜 characters and is ubiquitous at Chinese weddings.
“双喜临门” is a highly positive and celebratory phrase. It's used to express great joy and offer hearty congratulations.