When I start my computer this appears:
The computer works fine after that but I want to know what it is and how to get rid of it.
From the thread here I get that if you want it gone from your system it would require a reinstall using only primary partitions, as the mapper runs on logical partitions at startup.
In order to just prevent any text on startup, you could try passing the "quiet" option using grub. Edit /etc/default/grub and append quiet
into the speech marks after GRUB_CMDLINE_LINUX_DEFAULT
. Leave any existing value as it is i.e.
If your grub file has the line:
GRUB_CMDLINE_LINUX_DEFAULT="splash ipv6.disable=1"
it would become
GRUB_CMDLINE_LINUX_DEFAULT="splash ipv6.disable=1 quiet"
Then do sudo update-grub
to set the changes.