Learning Vim in 2014: Vim as Language is one of a series of posts by Ben McCormick about learning "modern" Vim. It takes the approach of describing Vim in terms of grammar, which I think was nicely encapsulated by the famous You Don't Grok Vi comment on StackOverflow.
On motions, Ben writes:
If you're a grammar nerd or remember your 8th grade english classes, you can think of these as transitive verbs that need to act on a direct object. These "direct objects" come in 2 forms, motions and text objects. Motions are the motion commands that you can use at any time to move around Vim.
Also, the post is on HN, so it would be nice to upvote it to encourage Ben to keep posting!