It means "I only use the Internet for short visits. I never stay online for long. I'm in and then almost immediately I'm out again." "In and out," and variations on that phrase, are a common way to saying "a very quick stop" or "a very quick visit." There is a chain of hamburger restaurants in the United States with the name "In-N-Out Burger." The name is promising very fast service. If you eat there, it won't take long, you can be in and out.

To me it is a bit strange to use the words in and out to describe internet use. I do not think you would say, 'I am in the internet'. Better 'I am on the internet'. So you could say 'I'm on and off', but people would probably also understand you if you used 'in and out'.