A colour space is a precise definition of which colours a video system can show — like a "palette" agreed in advance between the camera, the file format and the screen. Without one, the same numerical value of "red" would render differently on every device. With one, a colourist's reference monitor in Los Angeles and your TV in Berlin agree on exactly how saturated red 200 is.
The three colour spaces a product team actually needs to recognise: BT.709 is the HD-era standard, used for every regular HD streaming and broadcast in the world since the 1990s. BT.2020 is the wider, richer palette designed for 4K and HDR — it can show vivid neon greens and deep ocean blues that BT.709 simply doesn't have words for. DCI-P3 is the cinema-industry colour space, slightly narrower than BT.2020 but wider than BT.709; it's also the practical limit of what most current TVs and Apple displays can physically reproduce.
The reason this matters for a product: if you author content in BT.2020 and the viewer's TV only understands BT.709, the colours need to be translated down — done well, it looks fine; done badly, the green leaves go grey. Modern players handle most cases automatically, but the choice still drives mastering workflows, the bandwidth you need, and whether your library is "HDR-ready" or not.
