Reverb
The wash of reflections a space adds after a sound. It's what makes something feel like a closet, a hall, or a cathedral.
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What it is
Reverb is the pile of reflections a space smears onto a sound. They start arriving while the source is still playing, thickening it, and they keep decaying for a while after it stops, which is the tail you consciously notice. It is not echo: echo gives you separate repeats you can count, reverb blurs thousands of reflections into one decaying tail. That tail is the single strongest cue for how big a room is and what the walls are made of.
The controls that matter: pre-delay (the gap before the tail starts, which keeps the dry voice readable), decay or RT60 (how long the tail runs), size, damping (how fast highs die off), and the wet/dry mix. Flavors have names worth prompting with: room, plate, hall, spring, and convolution, which replays the measured impulse response of a real space like a church or a stairwell.
Gotcha: reverb eats intelligibility fast on narration. Use 20 to 40 ms of pre-delay, a short decay under a second, and high-pass the reverb return around 200 to 300 Hz so the low end does not turn to mud. Also, recorded reverb is close to permanent: dereverberation is a repair, not an undo. Track dry, add the room later.
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Add a short room reverb to this dry voiceover so it sits in the scene without going muddy: send it to a reverb bus at 100 percent wet, pick a small room or plate, decay 0.6 to 0.9 seconds, pre-delay 30 ms, damping high so the tail stays dark, high-pass the return at 250 Hz and low-pass at 8 kHz. Blend the send until the reverb is felt but not heard, roughly 10 to 15 percent, and keep the dry track untouched.