A crew collar is the traditional round, close-fitting neckline you'll find on most cycling jerseys. It sits right at the base of your neck and is by far the most common collar style available.
The main advantage of a crew collar is sun protection—since it sits close to your neck, it covers more skin than lower-cut options. It also layers well under jackets and over base layers without bunching or gaping. For most riders doing general road riding, gravel, or commuting, it's a solid, no-fuss choice.
The downside is that some riders find crew collars feel a bit restrictive, especially in hot weather when you're working hard. If you tend to run warm or dislike anything snug around your neck, you might prefer a V-neck or a jersey with a zip garage collar instead. But if you want reliable neck coverage and a classic look, crew is hard to beat.