A 1/4 zip jersey has a very short zipper at the neckline only—just enough to open the collar for comfort, but not much else. This design makes the most sense for thermal or cold-weather jerseys, where you're unlikely to need significant ventilation anyway. In freezing conditions, you actually want that front sealed up tight to trap warmth.
The minimal zipper also gives the jersey a cleaner, more streamlined look and shaves a tiny bit of weight, which appeals to riders who prefer a classic aesthetic. It's also common on casual or lifestyle-oriented cycling tops that prioritize style over on-bike adjustability.
Avoid 1/4 zip for summer jerseys or hot-climate riding—you'll miss the ability to dump heat quickly on climbs or during hard efforts. Save this option for when staying warm matters more than cooling down.