Platform is an industry term that means how readers already know about you. Not how readers will find you, or will hear about your book, but know about you now.
Platform is an essential part of a non-fiction book proposal. It's the first, and often the only, thing I look at when reading queries for non fiction. Only because if a writer doesn't have platform, the answer is no.
A lot of writers tell me they have blogs as part of their platform. I look at the blogs. If there are few or no followers, and no comments, the blog isn't platform. If no one is reading or following your blog, it's almost worse than not having a blog at all.
It takes a long time to build readership and encourage interaction with comments. You need to start doing it NOW, long before you query an agent.