From what I've gathered, it seems that it basically increases the length of Electronic Warfare (Debuffs) on enemy targets.
If you have no emitter, the duration of Electronic Warfare will be less than if you did have an emitter.
The Firewall Rating also comes into play here, if an enemies Firewall Rating matches your Emitter Rating, then the duration of the Electronic Warfare will be normal.
If an enemies Firewall Rating is less than your Emitter Rating, then the duration of Electronic Warfare will be longer.
If an enemies Firewall Rating is higher than your Emitter Rating, then the duration of Electronic Warfare will be less.
By default, the Command series of ships (Heavy Raider, Spectre and Hel) all have a higher Emitter and Firewall Rating than other ships. Meaning that, combined with an Emitter, can make Electronic Warfare attacks last so long, that you could potentially keep a constant debuff on an enemy. (Provided you have 2 of the same debuff.)
Hope this helps!