"At the spot" means what it sounds like. "At" is being used simply as a preposition showing where its object is spacially. Example: "The car is at the spot where I left it." I cannot think of other uses for this phrase. "On the spot" is often used as an idiom with a few different meanings. It can refer to something being done immediately. Example: "I bought the car on the spot" means "I bought the car immediately" It can refer to being put in a difficult situation. Example: "John was put on the spot when his wife asked him where he had been all night." John is in a situation where he has to quickly decide how to respond to what is a tense question from his wife. Thus he was put "on the spot." When a teacher calls on you in class, that is another example of being put "on the spot" in this way. "On the spot" can also refer to something or someone being at a specific location. Example: "When the car crash occurred, the paramedics were on the spot." They were there at the right time and the right place. Of course, it can also be used in the same way as "at the spot" as a simple prepositional phrase showing a position in space. Example: "The keys are on the spot where I left them" I hope that helps. Blessings