Can someone explain what this is?

[MjPioneer]

 

After all my ingition problems I routed the problem down to this soliniod looking thing. From what I have found the battery runs to it via power cable, and the ingition runs a signal wire to it. It is located in the jumble or relays behind the battery, if you can't tell it is the large silver rectangular thing. I need to replace it to get the ignition to work as it's suppose to. For the time being I have by passed to a push button for that neat ricer effect. What is it called and where can I find a new one?

[JeepcoMJ]

it's the starter relay.

[Harper]

.....and you can find one from Rock Auto parts http://www.rockauto.com for as little as $8.87, or as much as $37.79. But make sure you look under Cherokee 4.0; it's not listed under Comanche 4.0