journald is systemd's logging service. It is a utility for querying and displaying logs from journald. The logs for this service are stored in /var/log/journal/, which can accumulate quickly. If you have a system that you need to clean up quickly, getting rid of old journals is one way. There are 2 ways to do it:
By limiting the size of the journal:
Or going by the number of days:
Under normal circumstances, you should not have to touch these directories, as they rotate and space is freed on it's own.