Chroma is the strength or purity of a colour — basically, how far it sits from neutral grey. A high-chroma red is a fire-engine red: clean, intense, no grey in it. A low-chroma red is something brick-dusty and softened, like it's had a teaspoon of grey stirred through. Same hue, completely different feel.
Here's why it matters more than most people realise. High-chroma colours are loud and unstable. They reflect a lot of saturated light, they bounce that colour onto your ceiling and skin, and they swing hard between daylight and lamplight. Get them slightly wrong and a room feels like a soft-play centre. Low-chroma colours are calm and forgiving. They hold their character across the day, they sit happily next to other colours, and they rarely turn on you.
This is the whole secret behind why Farrow & Ball, Little Greene and Mylands feel so grown-up. Their colours are deliberately knocked back — greyed, dirtied, complexified. Mizzle, Pigeon and Treron are all low-to-mid chroma greens that read as sophisticated precisely because they're not pure. Even their reds and blues — Eating Room Red, Inchyra Blue — carry grey and black in the mix, so they feel deep rather than shouty.
The "but what about" question: don't confuse chroma with darkness. A colour can be dark AND low-chroma (Mole's Breath), or dark and high-chroma. It can be pale and high-chroma (a bright pastel) or pale and low-chroma (Cornforth White). Chroma is a separate dial from how light or dark something is.
Practically: if a sample looks fine in the tin but "too much" on the wall, it's usually too high in chroma, not too dark. Step down to something greyer in the same hue and the room relaxes instantly. For most homes, mid-to-low chroma is the safe, liveable zone. Save the high-chroma punch for a front door or a single bold accent — somewhere it earns its keep without running the whole show.