It means you are not making progress, that is getting stronger or leaner, or whatever you are trying to achieve. Changing your routine can be as simple as doing a different set and rep routine on the same exercises. For example if you have been doing 3 sets of 10 reps, change to 2 sets of 15 reps or 4 sets of 6 reps. Some people even change every workout, so one session you would do 2 sets of 15, next workout day 3 sets of 10 and next 4 sets of 6. Then repeat that sequence for the next 3 sessions. That way you are constantly "changing" your routine.