The End of Support Calendar shows Microsoft Operating Systems end of support dates, whether they are the end of mainstream support that updates features and cosmetic changes, or extended support, which ends security fixes and all Microsoft technical support other than old web pages.
Microsoft Office 2013 End of Mainstream Support
Includes all programs within this version of Microsoft Office, but the security problems apply in particular to software that receives information from the Internet: Outlook, OneNote.
The support date is for the last service pack for this version of Office, Service Pack 1. Support for this version of Office, without service packs, ended April 14th, 2015.
Mainstream Support refers to updated designs and features and other cosmetic changes and support provided by Microsoft that do not fall under security updates.