The following Globe and Mail citation tells you everything you need to know about this otherwise-inexplicable phrase:
Don't fear the cowbell. Once a herding tool for wandering beasts and later a percussion pariah, the humble rhythm tool is now hip. With the release of Maximum Cowbell, a 16-track collection of classic rock songs that employ its trademark hollow clank, the cultish fascination that began with a Saturday Night Live skit in 2000 grows.
The comedy bit involves a fanciful account of the seventies recording session that produced Blue Oyster Cult's soft-rock hit Don't Fear the Reaper. Christopher Walken portrays Bruce Dickinson, a producer intense in his belief that the track requires something extra specifically, "more cowbell."
Up to the challenge is Will Ferrell, the writer of the skit and a master at bumbling, occasionally destructive physical comedy. With the barnyard gong in hand, Ferrell bangs away at the cowbell urgently, although in perfect time.
The rest of the band is dubious, but Walken's character is insistent. "I gotta have more cowbell, baby!"
The repercussions of the sketch continue. Now part of the pop-culture lexicon, the catchphrase "more cowbell" refers to anything that needs a little something extra.
Brad Wheeler, "You gotta have more cowbell, baby," The Globe and Mail, April 6, 2006