There have been a number of big sales on apps and games in the Play Store as of late, but many of them don't include the US. Oh well, we tend to get preferential treatment from Google most of the time. Might as well let everyone else have some fun. Currently, Catan is down to $0.10, or whatever the smallest paid app price is in your local currency. It's not currently on sale in the US.
Catan is the mobile version of Settlers of Catan, a popular board game that I happen to be very bad at. Your goal is to build the best empire and outmaneuver your opponents, which can be humans or the AI. You earn points toward victory by having the longest road, the most towns, the largest army, and so on.
While the base game is on sale in most countries, there are still some in-app purchases for additional content that cost a few bucks. There are four expansion packs that can be purchased individually or as a single item for $10.