Tippet is the last 18–36 inches of your leader, and it's the only part the fish actually sees up close. Sold in spools by "X" rating: 4X is thicker (good for streamers), 5X is the everyday trout default, 6X disappears against sipping risers but breaks under stress.
Match tippet diameter to fly size with the rule of thumb: divide the fly size by 4 and round to the nearest X. Size 16 BWO → 4X. Size 22 midge → 6X. Light tippets sink emergers; heavy tippets pull dries.