What does Ubuntu base do?

The Ubuntu Base you are talking about is a way to aggregate the updates available for ubuntu-desktop.

I run Xubuntu - when updates are available for me - I see Xubuntu Base.

Further information on this can be found here - https://wiki.ubuntu.com/SoftwareUpdates#Expanded_presentation_of_updates

Previously Update Manager listed all updates available - it could, and can, at times be a long list. It's just a method of simplifying what you see when Update Manager starts.

No, it's not safe to skip the updates.

Linux is made up of a lot of different technologies which are being updated and improved by all sorts of companies and groups.

These changes need to be incorporated into linux distributions like Ubuntu or Redhat - they need to incorporate all these changes into their own distributions of linux to make sure it all works correctly and those changes will show up in Ubuntu base at the same time as other updates or included from other groups.

So Ubuntu base will hold the changes necessary to implement the updates coming from the wider linux world.

Is it safe to skip those updates?

Ubuntu base includes things like the Linux Kernel, so if you care at all about security these should be the most important updates to install.