Chip load (feed per tooth, f_z) is the size of the bite each cutting edge takes: the thickness of material a single flute removes in one revolution of the cutter. It has a healthy window on both sides. Too small a chip load and the edge rubs instead of cutting, generating heat and work-hardening the surface it is supposed to be removing; too large and the edge overloads and chips. Like cutting speed, the recommended value comes from the tool manufacturer for your specific cutter and workpiece material rather than from any generic table.
Its place in the arithmetic is the milling feed equation: table feed = RPM × chip load × flute count. A 4-flute end mill at 382 rpm and 0.002 in/tooth feeds at 3.1 in/min. The useful mental model is that chip load is the quantity you actually hold constant when conditions change — if the spindle speed drops because the machine cannot reach the computed RPM, the feed drops proportionally so each tooth keeps taking the same correctly-sized bite.