@Estel,
I will update the front page accordingly, the original pre-release/dev version of this app did check every 5 seconds to see what applications were running. However I have subsequently moved over to a DBus based approach which, while reducing reliability on certain apps, means it shouldn't drain your battery anywhere near as much as it would have.

To answer your question, the application does use RAM (all apps do), does use CPU whenever it detects an application being launched, and will drain your battery if you sit there opening and closing apps repeatedly. Unfortunately, due to the information that DBus transmits, AppLock has to check each and every app that is launched, so if you are in the habbit of launching LOTS of apps you may notice a small hit on battery life. Shouldn't be anything significant though.