"Pinnately compound" is part of botanical lingo for identifying plants by their leaf shapes. "Pinnate" means resembling a feather. "Compound" means that instead of one unified structure, a plant's leaf is really composed of multiple leaflets joined by stems.
Poison sumac has leaves made up of from 5 to 13 such leaflets. Notice that, while the exact number varies, it is always an odd number. That's because while most of the leaflets form matching pairs (one across from the other), there's always one lone leaflet at the tip of the compound leaf, which gives it the shape of a feather.