What is the Active Defence Multiplier (ADM)

Systems claimed by players may develop Strategic, Industry and Military indices based on the activity of players in the system and the amount of time that they have held their system. Increasing these indices indicates that players are invested in their systems and thus the systems are harder to capture. This is reflected in the Activity Defense Multiplier. The higher the various system indices are the longer it will take for hostile to disable affected structures when using the Entosis Link module.

Umm... can you explain it to me like I'm 5?

Nullsec is owned by player groups. Just as we came into possession of these systems, someone else might come and want them for themselves. Keeping ADMs high means we have more time to react when we inevitably get our systems toasted.

On the other hand, all of our systems have structures. If you see a system drop its ADM, please go and rat there instead of our staging that will most likely be sitting at lv6 all the time.

Higher ADMs also mean more ratting sites and better ratting sites. So your ratting in a system with high ADM levels literally does nothing to improve an already high system.

For more info please check out the official EVE website on sov or the wiki.