Creating a mask or selection key for compositing — extracting via chroma or luma key from footage. Foundation of all keying work.
You're sitting in the compositing suite in front of your footage: an actor against a green screen, or a VFX plate with a separable background. The first step isn't to key them out — it's to pull a matte. This means generating a binary or graduated mask that separates what should remain from what needs to go. Without a clean matte, nothing else happens. Everything else — color correction, keying refinement, integration — builds upon it.
In practice, this usually works via chroma keying or luminance keying. With green or blue screen (the classic setup), you use the background's color channel as an anchor: the keyer looks for all pixels within that color range and builds the matte from them — white for "keep," black for "discard," grays for transitions. In the keyer (Nuke, After Effects, etc.), you adjust the threshold values until the background is cleanly removed and the foreground (talent, object) remains intact. This is the "pulling" — the successive adjustment of keying parameters until the matte is right. With luminance keys (when you're separating by brightness, for example, with reflections or particles), it works analogously, just using the brightness channel instead of color.
The quality of your matte dictates everything else. Rough edges, missing hair details, "carpet effects" on the borders — all of this can be corrected later with matte finesse, erode/dilate, or spill suppression, but a poorly pulled base cannot be saved. That's why you look at the matte in isolation (as pure black and white), rotate it multiple times, and view it against different backgrounds. Only when you are satisfied with the matte's purity do you apply it as an alpha channel to your clean plate or your hero footage.
A pro tip: Even if the raw shot has a clean green screen, never pull the matte to 100% threshold — always leave small grades of softness. This prevents digital hardness on organic edges. And save your matte separately as a matte channel (not as an alpha directly on the plate) — this gives you room for adjustments later without damaging the plate itself.