Shortly put; "What you've done for the Sitecore Community".
Officially
This year, it was divided into 4 sections.
Motivation
What motivates you in your community work. And "I want to become MVP" is not going to impress anyone ;-)
Objectives (for next year)
What are your plans for community work in 2019next year. Regardless of MVP status.
Online Activities (this year)
A list of everything you've done, that we can track online. Open Source contributions, blogs, tweets, whatever it may be.
Offline Activities (this year)
And a list of what you've been up to, that isn't easily tracked online. User Group presentations, other offline activities.
And a few notes and considerations
The work you do in everyday life, working for your boss/employer - none of that needs to be part of your application. You don't get MVP for doing your job. It's fine you did a client workshop on SXA because a client asked for one; it's also fine that you built and delivered an Experience Awards winning site for another client. It's just not related to MVP status :-)
And scrambling onto every social media channel mid October when MVP season starts (but nothing going on for the entire year before October)... Good start - keep it up for the next round of nominations ;-)