A 2s5s10s fly is buying or selling the 5-year against the 2-year and 10-year in DV01-weighted size, so both the parallel shift and much of the slope are hedged. What is left is whether the middle of the curve is rich or cheap.
The fly spread is quoted as 2 x belly yield minus the two wings. Traders use flies to trade the expected timing of the policy peak, since that is what bulges the belly.
Example: 2-year 4.50%, 5-year 4.15%, 10-year 4.30%. The fly is 2 x 4.15 - 4.50 - 4.30 = -0.50%, or -50 bp. If the belly cheapens 10 bp relative to the wings, the fly moves to -30 bp.
Related: humped-yield-curve, key-rate-duration, curve-steepener, dv01