Resources or Rss, are the materials and supplies needed to develop a Kingdom.
Food is a resource that can be collected from Farms, obtained through raids, captured Farmlands, and bought in the shop.
Its main use is to train and upgrade troops, to repair Relics, and to Fuse Titans.
Gold is a resource that can be collected from Gold Mines, obtained through raids, captured Gold mine lands, and bought in the shop.
Its main use is to construct and upgrade buildings, and to craft spells.
They are used to directly battle other players or raid/capture their lands: For lands, a raid costs 3 Portal Stones, a capture 6 Portal Stones. A raid directly on another players Garrison is also 6 Portal Stones.
Gems are the premium resource in the game, and can be bought in the shop or gained through the completion of Quests. Use them to obtain anything in the game - Gold, Food, Portal Stones, Titans, Relics; or to reduce building/upgrading times.