Chocolate Dip gracefully shows us how it's done
Sock posts are a simple meme in which the Tumblrpony receives a request to appear wearing socks. The length and style of the socks varies from blog to blog, but the overall effect is generally aimed somewhere between 'adorable' and 'seductive'.
As is generally the case with meme posts, these images are almost always non-canon, allowing the blog authors to put socks on some rather unlikely victims...