What does you must be my backyard cause i really dig you means?

The phrase "You must be my backyard cause I really dig you" is a playful and flirtatious way of expressing romantic interest. It uses a pun; "dig" refers both to liking someone and to the act of digging in a backyard. Essentially, the speaker is complimenting the person by suggesting that their attraction is as natural and fundamental as a backyard is to a home. Overall, it's a lighthearted way to convey affection.