An all-season jersey is designed to handle a wide range of temperatures through smart layering. It's typically a midweight piece with ventilation features like underarm zips or mesh panels that let you dump heat when it's warm, plus enough coverage to stay comfortable in cooler conditions when paired with a base layer underneath.
This makes it a great option if you're just starting out, traveling, or simply don't want to manage a full cycling wardrobe. On a cool morning, wear it over a thermal base layer; as the day warms up, strip the base layer off and open the vents.
The trade-off is that an all-season jersey won't match the cooling power of a dedicated summer jersey on scorching days, and it won't replace a proper thermal jersey when temperatures really drop. Think of it as a solid everyday choice that covers most riding conditions—it just needs a little help from layering pieces at the extremes.