When trading the actual underlying security, spot instruments are ones that you can have a physically delivery of immediately - "on the spot". As for futures, you pay a predefined price to have a good delivered to you at a predefined date in the future, let's say 1 year.
Since you won't be purchasing the good immediately, there is a cost of carrying. That is the cost of holding the physical goods for 1 year until they are delivered to you. That would include insurance, interest, etc.