If EURUSD Spot is currently trading at 1.21, then trading today for the usual settlement of EURUSD on today+2 would be at 1.21. If you wanted to trade for immediate exchange of EURUSD, what would be the right rate? 1.21? Not really; if EURUSD was expected to go up or down between today and the spot date, I could arbitrarily profit from taking one side or the other.
So just as the market trades the interest rate differences after spot using FX Swap points, it also trades them before spot too. There are two common trades before spot:
ON: Overnight, trades between today and tomorrow. So instead of having USD overnight tonight, you have EUR (or vice versa).
TN: tom(orrow)-next, trades between tomorrow and the next day (i.e. spot for t+2 currencies).
In order to construct an outright FX price for today, you need to buy ON, buy TN and buy Spot, or sell all three. This way the ON back exchange would cancel the TN front exchange, the TN back exchange would cancel the Spot exchange, and you'd be left with a net exchange today at a known price.
I'm not aware of intraday FX forward trades, so presumably 'immediate' is treated as 'today', but given global trades and differences in posting deadlines I wouldn't be surprised if there was a way to trade between the shortest cash posting deadline and the EoD collateralised worlds for an even more immediate requirement.