Terrestrials are land insects that end up in the water by accident: ants blown off streamside brush, beetles that miss a leaf, grasshoppers that jump short. From June through September, with the peak in July and August, this is often the most reliable trout food on a stream - no hatch to time, just bugs falling all afternoon when the water sits in the 55-75F range.
Trout learn that a terrestrial is a big, helpless meal, so they move for it. Look for fish holding tight to grassy banks, under overhanging brush, and in the foam lines that collect drowned bugs. Wind is your friend here - a breezy afternoon knocks more insects onto the water.
Match the bug you see. A foam beetle or fur ant covers the small stuff and gets eaten in slow, clear water where fish inspect their food. When you want a high-floating, easy-to-see attractor, tie on a hippie stomper or a Chernobyl ant, or run a Dave's hopper along the bank. Fish them dead-drift, but an occasional twitch - a struggling bug - can trigger a take.