I already posted this question on StackOverflow. I got a suggestion to go for a better answer here.
To make the question more concise I'm interested in an introduction to digital filters, any resource is good.
But first I will be satisfied if someone could point me to some resources explaining this filter (used in Android):
What I'm interested is the way the filter's parameters are chosen. While I can copy it without thinking, I guess I should understand the basic concept/idea behind before using it.