I haven't seen this message from Update Manager before, and I have not been able to google up any info. What will be installed when I click Install? (Clicking "Description of Update" got me nothing...)
It's simple, really. Ubuntu 14.04 is now out, so what the Ubuntu developers did was make an update that makes Ubuntu 12.04 compatible with all of the same hardware Ubuntu 14.04 is compatible with in the form of an update. This makes Ubuntu 12.04 work with more computers, but if you're already running Ubuntu 12.04 just fine and all of your hardware is working properly, then this update probably won't do anything beneficial to you. I still strongly recommend that you install it anyway, even if all of your hardware is working right.
OK, I bit on it and clicked install. It then gave the desired info via a link to this web page: https://wiki.ubuntu.com/1204_HWE_EOL
It looks mandatory to me since the link (https://wiki.ubuntu.com/1204_HWE_EOL) said:
Starting Aug 8, 2014 systems running 12.04.2 HWE, 12.04.3 HWE, or 12.04.4 HWE will no longer receive software updates for the kernel and, if you're running it, the graphics stack.
So there will be no more update for those who are running old HWE stacks (e.g. fresh install from 12.04.x ISO)...
I think the Update Manager should give us more info besides that simple "Install" button hanging over there.
I can't comment on an answer because I don't have enough rep, but my husband did that update on Precise and it completely hosed up his system and he had to do a total re-install. So my advice would be if you are running Precise 12.04, don't do that upgrade. We spent hours trying to repair his system because of it.
It sure would be great if, instead of just putting the message and an install button in the update center, they would leave a link to let you know what it was all about.