A tyre system with no inner tube: sealant closes small holes from the inside. It lets you run less pressure without pinch flats — on a road bike about 0.5 bar below your tubed value.
Without a tube the lower limit that pinch flats otherwise impose disappears: you can run noticeably softer, and that is exactly where grip and comfort live. Sealant closes small punctures from the inside straight away, often before you even notice them. The most common mistake is running tubeless at road-bike pressure and throwing the whole advantage away; the second is going so low the tyre squirms in corners and the rim bottoms out over edges. Work down in steps of 0.1 to 0.2 bar and write down the value where grip and puncture protection line up. Two things get overlooked: a spare tube stays mandatory, because not every cut seals. And in licensed cyclocross racing a 33 mm tyre-width limit applies — tubeless tyres on wide rims often measure wider than printed, so actually check before the race.