This is a really nice relay setup, as it's 100% waterproof, and can handle up to 40 amps. This waterproof relay also has a single pole double throw which means it has a normally closed contact and an normally open contact.
If you don't know what a relay does, here is the short course. Basically a relay uses a low power electrical signal to switch a high power set of contacts. This is done using an electromagnetic coil on the low power side. When the coil is powered it will pull one set of the high power contacts together, and the other set apart. For example if you have a small switch on your handle bars it probably can't handle all the current that is needed for things like the head light or auxiliary driving lights. It will either get hot and burn up or create a significant voltage drop to the point where the performance of the lights will suffer. The solution is to use a relay...The small switch closes the circuit for the coil which in turn pulls the high power contacts closed and the lights turn on with all the current they need. Simple.
This relay is better than most, first because it is waterproof. Second, because of the connectors used in the housing, they are not your average "spade" connector, these are designed with a spring lever that ensures positive contact even under extreme conditions. Third, the connector latches to the relay body which keeps it from disconnecting at an inopportune moment. And fourth, it has a handy mounting bracket built in to make installation a snap (if you don't have room for the bracket it is removable, also good for servicing).
Comes as a kit with connectors, all the bits and pieces, and instructions you need to make a splice free install. (Note: The crimp terminals are open barrel type and it is best to use an open barrel crimp tool; if you don't have one, we've got a good and inexpensive one here. It is possible to get decent crimps with other tools, it's not recommended, it's a little frustrating, but if you are dedicated and operating on the cheap it can be done)