On-the-spot means done at once, right there, at that place. An on-the-spot fine: the officer charges you now, on the street, not later by letter. An on-the-spot decision: you decide in that moment, without going away to think. An on-the-spot check: someone looks now, here. It comes from the phrase on the spot (at that exact place, or at that exact moment), written with hyphens so it can sit before a noun. Though actually, to put someone on the spot is a nearby phrase with a different job: it means to ask them a hard question in front of people, so they must answer with nowhere to hide.