The term, which typically means an animal who feeds on plants alone, is meant to indicate a man's disinterest in "flesh" — in other words, they're not into chasing women or accumulating material goods. Almost half of the men surveyed by a Japanese research firm described themselves as "herbivores."
Traditionally, Japanese men spend most of their days at work or socializing with colleagues, and little time with their families. The media is pointing to disillusionment with the troubled economy as the cause for a growing rejection of this typical workaholic role for men. Instead herbivores typically work part-time and spend their free time alone or on the Internet.
I have to say that finding a new way to define what it means to be a man is much more refreshing than the other Japanese trend the media has latched on to — falling in love with body pillows.