The fed funds rate anchors everything: mortgage rates, margin interest, Treasury yields along the yield-curve, the discount rate used to value stocks, and the swap-rate in forex.
Higher rates make cash more attractive and tend to compress valuations; lower rates do the opposite. Futures on the rate let traders see what the market expects the FOMC to do.
Example: the target range is 5.25% to 5.50%. Fed funds futures price a 70% chance of a cut to 5.00% to 5.25% at the next meeting.
Related: fomc, rate-hike, yield-curve, carry-trade