The 26‐inch refers to the wheel diameter — a very common size for mountain bikes and bikes for teens/adults. It’s suited for riders typically about 5′2′′ to 6′0′′ tall, though this also depends on the frame size and your inseam.
26′′ wheels provide a good balance of maneuverability, strength, grip, and control — great for city riding, light trails, and everyday use.