Anything that's untied is freed, loosened, or unfastened from something that was restricting it. Is your shoelace untied? You may want to lace it so you don't trip.
Are your helium-filled balloons untied? Hang on to them, or they'll float away! Did you leave your boat untied at the dock? Careful — it might drift out to sea. There's also a figurative way to be untied, like when your untied thoughts drift freely through your mind before you fall asleep. When you're reading or writing this adjective, it's easy to mistake it for united, which means "joined as one."