Its about giving back to the vCommunity which can include helping on here and writing blogs etc. See the vExpert site for more information: https://vexpert.vmware.com/ 

"If you are interested in becoming a vExpert the criteria is simple. We are looking for IT Professionals who are sharing their VMware knowledge and contributing that back to the community. The term "giving back" is defined as going above and beyond your day job. There are several ways to share your knowledge and engage with the community. Some of those activities are blogging, book authoring, magazine articles, CloudCred task writing, active in Facebook groups, forum (VMTN as well as other non VMware) platforms, public speaking, VMUG leadership, videos and so on."