What is this glassware between flask and condenser to collect liquid?
This glassware is used to separate /remove the light phase from a biphasic (immiscible) liquid mixture.  This can be used primarily to enhance liquid-liquid extraction from the light phase to the denser phase. As a mixed distillate collects,  the denser phase will eventually siphon back through the side-arm to the flask below and the light phase remains in the collector.