A free spin is essentially an offer that a casino gives to its players to spin the reels free of charge and have the opportunity to win real cash. The free spins may be given to both new and current customers, and it's one way to reward and encourage them to continue betting.
Cuba is an emblematic Caribbean country in many ways, and despite difficult political relations with the US, it still attracts curiosity abroad. The communist regime banned all forms of gambling decades ago, and the ban includes online casinos. In practice the ban is not strictly enforced, and some international mobile casinos do accept Cuban players.