i don't use it but iblacklist allows you to put users on who you don't want to hear from. for instance a stalker ex bf or something (assuming you are a hot blonde

) you put his number in, and if he calls or texts you, he either just gets a busy signal, voicemail, or whatever you assign to it. the whitelist is for lets say if you are in an important meeting but are waiting for an important call from your friend about something you really need to know at the moment, it will stop all calls to you EXCEPT from that one person you list (or whoever else on the whitelist).
i am quite sure the free version is rather limited (you can only put a few people in for each) but the full version lets you do a lot. hope that helps.