Kicker is an accelerant for CA glue, it comes in handy when you don't have to hold 2 pieces together for an extended period to let the CA dry. You can either glue the pieces together and spray the kicker, or spray the kicker on your pieces to be bonded and once the CA hits it, your bond is complete.

Thin, medium, and thick all have their place. Majority of what I use is medium. It's gap filling but yet viscous enough to flow into crevices. Generally ill use thin to tack, then come back with medium or thick CA. Or use it between but joints, it has great capillary action to get sucked into the tiniest of spaces. Lastly I use thick for high strength areas or when you need to position a piece in place before the glue sets(most thick CA will give you plenty of time to get things aligned and clamped if need be before it's too late.