Understanding gain and decision trees

I am trying to understand the gain formula for decision trees and I am currently going off of this slide deck: http://www.site.uottawa.ca/~stan/csi5387/set1.pdf

(Look at slides 25-30).

I am not quite following this information as the deck isn't the best put together. Do you know of any sources that can explain the same topic in a way I can actually understand?

Thanks