A club fit (sometimes called slim fit) jersey is athletic but not skin-tight—it follows your body shape without squeezing. Think of it as the sweet spot between a race jersey that feels painted on and a relaxed jersey that can flap in the wind.
Club fit is the most popular choice for good reason. It works great for club rides, sportives, and regular training where you want to look the part and avoid excess fabric catching wind, but don't need every last aerodynamic advantage. It's also more forgiving for different body types than race fit.
If you're not racing or doing fast group rides where seconds matter, club fit is likely your best bet. It gives you good aero properties and efficient moisture management while keeping you comfortable for hours in the saddle. Most cyclists should start here.