Spontaneous casting refers to casting spells from a spellbook without previously allocating a spell slot to the spell. A spell slot of the same level as the spell — after considering metamagic — is still consumed by such a casting. Sorcerers and bards can cast any spell in this manner, and as a consequence these classes never allocate spell slots to specific spells. Clerics can spontaneously cast the "cure" and "inflict" spells (from minor to critical wounds), which will consume a readied spell slot not assigned a domain spell (the leftmost such slot of the appropriate level, when viewing the spellbook).
To set a spontaneously cast spell for a cleric, the player must drag the spell from the spellbook to a quickslot. These slots will have their number of uses (top-right corner) displayed as "S", indicating that the slot represents spontaneous castings. (Technically, the "S" designation only appears when the cleric has no memorized uses of the spell remaining, but this is usually the case as players never need to explicitly memorize spells that can be spontaneously cast.)