libsysstat is a library to query system information like CPU and memory usage or network traffic. Conceptually it's roughly similar to libstatgrab, a third-party library used by LXQt as well. It is maintained by the LXQt project but can be used independently from this desktop environment, too. As for LXQt libsysstat is used by plugin-sysstat of lxqt-panel and hence an optional dependency of the latter.