Working with journalctl

journalctl is tool that interfaces systemd-journald

journalctl
Shows the complete journal

journalctl -u <unit>
Shows information about specific unit (use Tab completion)

journalctl --dmesg
Shows kernel messages, dmesg is the old Linux command which reads the kernel ring buffer. And the kernel ring buffer as the name suggests is showing you kernel messages or it can be very specific.

Combined filters can be used:
journalctl -u crond --since yesterday --until 9:00 -p info
-p is about priority which is indicator of seriousness of message, in this case you'll see everything, --since/--until - time range