Shots recorded without sync to dialogue or music — ambient footage, reactions, movement for post. Shot parallel to dialogue takes for editing flexibility.
You're shooting the dialogue takes, the actor is sitting at the table and says their line — but at the same time, you're noting to yourself: I still need a wild picture of them listening. Or of her hand lifting the glass. That's the core: Wild pictures are material without sound sync, which you film parallel to the dialogue scenes and will need later in the edit to resolve the montage.
On set, it works like this — after the clean dialogue take, you say "One more wild in" or "A reaction without sound." The actor holds the movement, the emotion, repeats it without speaking. Camera rolling, no sound is recorded (or the sound assistant has a silent slate). This gives you the flexibility you need later in the edit: You can cut faster, can place a reaction differently, can move away from the speaker's perspective. Wild pictures are your escape route from dialogue captivity.
The classic use cases: One character listens while another speaks — instead of seeing them in the original every time, you look at the wild of the reaction. Or you need a movement that wasn't clean in the dialogue take — picking up a glass, standing up, lowering your gaze. A wild can be shot in two minutes and save you three dialogue takes. Also for dissolves, for cutting rhythm, for montage gestures — opening a door, looking out the window, flipping through paper — everything is shot wild.
Practical tip: Don't shoot wilds as an afterthought, but as an integral part of your take. If the dialogue take was good, shoot the wild immediately afterward. Same lighting, same camera angle, same emotional moment. Otherwise, in the edit, you'll pay with visible jump cuts or lighting changes. For long scenes with a lot of dialogue: shoot multiple wilds from different angles. The editor will thank you if they have six options instead of one. Also for music videos and montage sequences: wilds are your primary tool — pure picture material without a temporal link to audio.