Tumblr submit feature is a very popular function (initially only for pro services) that allows people to add content to your blog, of course, you can deny or approve the content submitted, if approved then this content will turn into a post.

Go to your SETTINGS and the select the blog you want to enable the submissions. Find the Submissions option and click on it. Save and it’s done! Now your submissions page is: http://username.tumblr.com/submit


P.S.: You can change the submit page title and choose what type of post people can send you.