The term "10x" engineer refers to engineers who are 10 times as productive as the average engineer, largely because they create better abstractions, solve metaproblems, and make maintainable code (they generally can't type 10x as fast).

Resig is at least 100x in this department but if you really tally up the impact of JQuery etc. it's probably even more than that. Then there are the (infinity)x guys like Torvalds.

I assume that is referring to Paul Graham's assertion that a good developer isn't just a little better than an average developer, but is instead orders of magnitude better.