This group of words is defined as a method of determining a outcome by chance. The cast part of this phrase is the means of using something other than human opinion to make a decision; much like flipping a coin. The lot part of this phrase is the object, thought, or action you or someone else is trying to make the decision over.
Me and my friend what to go out and have some fun, but I want to do something totally different than my friend wants to do, so instead of fighting about it and going our separate ways for the evening; mad at each other, we deside to cast lots; to but it in todays terms, we flip a coin, or do paper, rock, scissors. We are able to not lose face with each other and still go out, now to be fair the one who won, should let the other party deside next time what is to be done.