interstate commerce. You may conduct business across state lines but, for example, you don’t plant yourselves, your employees, or your inventory solidly within another state’s borders for extended periods of time.
What does it mean to qualify in another state? If a business must qualify as a “foreign” corporation, that means the company must file some paperwork, pay some fees, and then pay taxes each year to the other state. Here's an article summarizing all this and you can learn about each state’s rules for qualification at the state’s website for its Secretary of State.