The message is suspect. Please see http://forums.xfinity.com/t5/Email-Web-Browsing/Share-app-no-longer-available/m-p/2868913#M175525
and Comcast Email Phish or Legit? How to Tell (and rules for posting) for more information.
I have been contacted by Xfinity to update or download the Share App or it will be gone by April 1st. What is it, didn't know I had it and what if I don't download it.
There is an app called "XFINITY Share" that will be decommissioned soon. It was likely used at some point my someone in your household (the emails are targeted to former users of the app). There is a new app (with some different functionality) called "X1 Photos" that is taking its place (allows you to stream photos to your X1 TV).
The only reason you would need to download or upgrade the soon-to-be decommissioned Share app is that it allowed customers to upload photos and videos they wanted to appear on their X1 set-top boxes. We just don't want users to lose personal content they may care about. If this doesn't apply to you, you can safely ignore the email.
Hope this helps clear up any confusion.