It’s simply a source device for storage on your VPS. You can read up on ploop here:
“ploop is a kernel block device, similar to the traditional loop device (which is controlled by losetup(8)) but with more features added, such as dynamic disk space allocation, stackable images, online resize, snapshotting, and live migration helper (write tracker). This manual page describes the ploop user space tool which is used to perform various operations related to ploop devices and images.”
If you are on a VPS, then /dev/ploop15234p1 is where the storage for root (/) is coming from. All VPS providers provide root storage in different ways and this is one of them I guess.
So instead of being concerned about what that device is — it’s clearly storage assigned to your account — I would instead look through your system from root (/) and see what is taking up space. The simplest way is to run a du (disk usage) command like this:
sudo du -sh /*
That will inventory the disk usage in all directories and files on your system. My guess is whatever is taking up space would be under /var/. If my hunch is right run that command again like this:
sudo du -sh /var/*
Or replace /var/ with whatever directory path seems bloated. But would be willing to say that /var/log/ is what’s filled up with logs taking up space.
How to deal with that is out of the scope of this question, but you will most likely have to delete logs. Might be able to Gzip them as well if you can free up enough space.