If something is interstate, it happens between different U.S. states. For example, you can drive on Interstate 80 from downtown San Francisco, California to Teaneck, New Jersey. Yep, that's a trip.
If you're driving from Rhode Island to Illinois, you can call it interstate travel, or an interstate road trip — you're going from state to state. An interstate highway connects different states together, like I-95, which is an east coast interstate highway that stretches from Maine to Florida. For short, you can call a road like this an interstate. This 20th century word combines inter-, "between," and state.