A plugin can be temporarily bypassed, i.e. disabled so that it has no effect on the output. The bypass state is a toggle: to disable or reenable the selected plugin, use Plugin/Bypass, or choose Bypass from the plugin context menu.
It's also possible to solo a plugin, i.e. bypass all plugins except the selected plugin. This can be useful for isolating problems. Solo is also a toggle: to enter solo mode, use Plugin/Solo, or choose Solo from the plugin context menu. Choose solo a second time to exit solo mode; this restores the bypass settings that were in effect when solo mode was entered.
Note that Bypass works in solo mode; this allows you to solo a plugin, and then add the other plugins back into the chain one at a time, by un-bypassing them.
Also note that in version 2, bypass/solo doesn't change the routing, therefore it's possible to get unexpected results.