Portal Frackers are items that temporarily double a Portal's Hacking output. When deployed, a fracker lasts for either 10 minutes, or 150 hacks total among all agents, whichever comes first. Frackers are not Portal Mods, they do not occupy a mod slot and can be applied to a fully modded Portal. Frackers are never dropped from hacking, they can only be bought from the Ingress Store.
When a Fracker expires, it removes 50% of each Resonator's remaining health (this a second fracker would reduce that to 25%). This is occasionally used to tactical advantage, to reduce a well-guarded portal's health, as Fracker damage does not send an alert to the Portal's owner(s) in COMM.