FC 26 Coins are the in-game currency used in EA Sports FC(TM) 26 (the successor to FIFA). You spend coins to buy players, consumables, manager cards, position modifiers, and to complete SBCs faster. If you are building a competitive Ultimate Team, coins are basically your oxygen: they let you test new metas, flip squads between Weekend League runs, and react quickly when a promo floods the market.
[Bottom line] more coins = more flexibility, fewer grindy hours, and far less FOMO when content drops.