Meh means rubbish. It means boring. It means not worth the effort, who cares, so-so, whatever. It is the all-purpose dismissive shrug of the blogger and messageboarder. And it is ubiquitous.
No one is quite sure where it comes from. Graeme Diamond, principal editor of the new word group at the Oxford English Dictionary, says it's not yet suitable for the OED, but he does have a "meh" file, and the first recorded print usage occurred in the Edmonton Sun newspaper in Canada in 2003: "Ryan Opray got voted off Survivor. Meh." He thinks, however, it sprang into common usage from the Simpsons.
Some credit the 2001 episode Hungry Hungry Homer with the first use of "meh" as a dismissal, when Homer asks Lisa and Bart if they want to go to the Blockoland theme park and receives the answer, "meh". But the Language Log website notes a 1995 episode in which Bart dismisses Marge's discussion of weaving with a "meh".
Some amateur etymologists reckon meh is derived from Yiddish, pointing to a 1936 song that uses it as the sound of a goat bleating. A poster on some net thing called it a "Yiddish interjection used to express disdain that borders on apathy", but did not source it.
It is not a word to use lightly. Meh.