
Your legs are open and your feet and grips control them. Keep your feet between you, then sweep or stand up.
Open guard covers every guard where your legs are not locked: butterfly, De La Riva, spider, lasso, collar-sleeve and more advanced ones like X-guard. Instead of holding them in place, you use your feet, shins and grips to steer them, off-balance them and keep them away from your chest.
The rule shared by every open guard is to keep something between you, a foot, a knee or a frame. Once that is in place, chain off-balances into sweeps, take the back, or stand up into a takedown. Pick one or two guards to learn first rather than trying them all.