lxqt-policykit is the polkit authentification agent of LXQt.

polkit is a software framework to handle privileges of processes. In LXQt it is e. g. used to extend the privileges of the GUI tools of lxqt-admin. 
These are launched by a regular user. But in order to apply the settings they deal with root privileges are needed and acquired via polkit. 
Among the various components of polkit the authentication agent is the one to query the user for credentials by dialogue windows like this one of lxqt-policykit.