Understanding Jean-Paul Sartre phrase “existence precedes essence”

http://en.wikipedia.org/wiki/Existentialism

The early 19th century philosopher Søren Kierkegaard is regarded as the father of existentialism.^[5][6] He maintained that the individual is solely responsible for giving his or her own life meaning and for living that life passionately and sincerely,^[7][8] in spite of many existential obstacles and distractions including despair, angst, absurdity, alienation, and boredom.^[9]
 
Source: http://atheism.about.com/od/existentialistthemes/a/existence.htm

Hammers are made by people for certain reasons — in a sense, the “essence” or “nature” of a hammer exists in the mind of the creator before the actual hammer exists in the world. Thus, one can say that when it comes to things like hammers, essence precedes existence.