I must disagree with jessamyn. And, apparently, everyone else. Best of the web refers to the quality of the subject of the post and, as such, is a succinct expression of what qualifies as a good post. What qualifies for best of the web? That's more fully explained in the post guidelines.
Originally, the post page stated: "Just to be clear: the point of metafilter is to find the best and most interesting of the web to share with others, not things described here." Now it just says: "Found something cool on the web and want to share it with everyone else? Great!" The phrase "best of the web" is not found on either the about page or in the guidelines.