Currency, as used in this context, is a medium of exchange for goods and services.
Like many other games, FarmVille 2: Country Escape has some type of currency. Some can be bought through microtransactions or in game payments using cash. The three types of currency that can be bought this way are coins, Keys, and Stamps.
All types of in game currency can be acquired without spending cash. These ways include Quests, neighbors, Foraging, or trading for them using other in game items. For instance, Ribbons can be traded for Barn Nails or stamps at the Prize Tent. These ways take longer and require patience to save enough.