Spot is the plain purchase of one currency against another at today's rate, with the actual exchange of money happening on the spot-date. Everything else in the market, from a fx-forward to a currency option, is priced off the spot rate plus the cost of carrying the position to a later date.
Retail forex is spot in name only. Almost no retail account ever takes delivery; positions are rolled indefinitely through tom-next, which converts the trade into an open exposure financed by the interest differential.
Example: you buy EUR 100,000 against dollars at 1.0840 on Monday. Settlement would be Wednesday: EUR 100,000 in, $108,400 out. A retail broker instead rolls the position each night and applies a swap-rate rather than moving any principal.
Related: spot-date, tom-next, rollover, fx-forward