Open guard is an umbrella term for any bottom guard position where your ankles are not locked. The family includes butterfly guard, De La Riva, spider guard, X-guard, and many more. Each variant uses foot and shin placement to control the passer's hips and legs rather than body weight and closed legs.
Because your legs are not locked, the passer has more freedom — and so do you. Open guards are more dynamic and require more active foot and hip work than closed guard. They are common at higher levels and in no-gi, where closed guard is harder to hold.
Beginners often encounter open guard when opponents break their closed guard and step back. Learning to sweep and recover is the first open-guard priority.