A 3x6 FRA covers a three-month period starting in three months. If the agreed rate is 4.2% and the reference fixes at 4.7%, the buyer receives the 0.5% difference on the notional for the period, discounted back because FRAs settle at the start of the period rather than the end.
FRAs are the single-period building block of an interest-rate-swap, which can be decomposed into a strip of them. Treasurers use them to lock in the cost of a known future borrowing.
Exchange-traded interest rate futures serve a similar purpose with standardised dates and central clearing, and have absorbed much of the volume that once traded as FRAs. See forward-contract.
Related: interest-rate-swap, forward-contract, overnight-index-swap, futures-contract, swaption, dv01