It means that the supplements were created with Good Manufacturing Practices. Generally, all that means is that they were following the rules when they were creating the supplements en masse in a lab: from the types of ingredients included in the supplements to the amounts and the ratios, good manufacturing practices were followed.
Seeing this label on a supplement is generally a good thing. It means that people are following the rules while creating the supplements and usually is a point towards the supplement being safe rather than not. Good manufacturing practices tend to make it harder to get a dosage wrong or poison someone with unhealthy ingredients.