End of Standard Support and End of Life
From Wikipedia:
"End-of-life" (EOL) is a term used with respect to a product supplied to customers, indicating that the product is in the end of its useful life (from the vendor's point of view), and a vendor stops marketing, selling, or rework sustaining it. (The vendor may simply intend to limit or end support for the product.)
With this in mind, we can deduce that end a Ubuntu version in EOL means that it won't get supported in any way by Canonical, that is, it won't get any updates or support.
But what about a product that is in End of Standard Support? According to this article made by Canonical,
As Ubuntu LTS releases transition from the standard, public maintenance window, Extended Security Maintenance (ESM) is available for organisations with their workloads on Ubuntu LTS releases to maintain that stability and continuity, ensuring system security and compliance. With the end of its five-year standard support window in April, Canonical is announcing the availability of ESM for 14.04 starting 25 April 2019.
ESM for 14.04 provides ongoing kernel security fixes through a secure, private archive for three years until April 2022. This continued protection from major security vulnerabilities enables development and operations teams to plan and manage application upgrades in a failsafe environment.
ESM is available as a feature of an Ubuntu Advantage subscription.
End of Standard Support is a newer term Canonical have been using to replace End of Life (for LTS releases) when the reach the end of their 5 year freely supported window (ie. when LTS transitions to ESM).
Ubuntu 14.04 LTS reached EOL using older terminology 2019-04-25 (http://fridge.ubuntu.com/2019/05/02/ubuntu-14-04-trusty-tahr-reached-end-of-life-on-april-25-2019-esm-available/) and the Fridge post reflects that (using language in header consistent with prior LTS EOL's). Canonical though wanted to use the term End of Standard Support.
Users of Ubuntu 14.04 LTS had the choice of release-upgrading to Ubuntu 16.04 LTS to still receive updates, or start paying for security upgrades and switch themselves to Ubuntu 14.04 ESM (Extended Support) though Ubuntu Advantage.
Ubuntu LTS (desktop; Unity for 16.04 LTS, GNOME for 18.04 LTS), server and main products come with 5 years of free support, however flavors of LTS products only generally have 3 years of such support (the exceptions are shorter lives, not longer). Thus whilst Ubuntu 16.04 LTS is still covered in the initial 5 years of support (see http://fridge.ubuntu.com/2019/03/01/ubuntu-16-04-6-lts-released/), Lubuntu 16.04 LTS along with all other flavors (excluding Ubuntu Kylin 16.04 LTS) are EOL as they only came with 3 years. If you installed a 16.04 LTS flavor (such as Lubuntu) your desktop, programs or anything added from 'universe' will be unsupported, as can be seen by running ubuntu-support-status where you'll have a good number of packages in the unsupported category.
In essence, you should read your release notes and take note of what it states. Ubuntu Studio 18.04 was not a LTS release, so assumptions should not be made (ie. seeing 18.04 doesn't mean LTS!). People using Ubuntu Studio 18.04 need to add a PPA for extended support for that flavor as has been covered in numerous posts to get their 3 years (flavor) of support.