A spot counter is a counter that can be used immediately from spells, relics, and equipment, rather than relying on an ability on a champion (since champions generally take time to get AP and move to where the effect is needed).
Specific types of spot counters include spot cleanse, (such as from Vial of Holy Water and Cleansing Emerald), spot equipment counters (such as Disarm (spell) and Diamond Mace) and spot healing (such as from Griefbearer's Fountain)