Usually when installing ESM, you'll setup an archive location, size and job.  Once that location is full on space, archives will no longer run and you'll see those messages that the archive space is full.  You can either delete older archives or move them to other storage to save them.  There's more information about archives in this guide starting at page 155: https://www.microfocus.com/documentation/arcsight/arcsight-esm-7.7/pdfdoc/ESM_ACC_UserGuide_24.1/ESM_ACC_UserGuide_24.1.pdf 

By archives do you mean all the events that have been received in ESM since it was installed?

Did I understand correctly that I don't need this data, so I can just get rid of it?

Yes, the event data would get archived on a daily basis from when you have setup archives.  It would be your choice if you decide you need this archived data or not.  If you never plan to go back that far and search the older archived events and if you don't need it for any compliance reasons, then you probably would not need it.  Again, that would be something for you to determine.