The tests attempt to determine implicit association - If two concepts are closely related in a person's mind, such as "white person" and "friendly", they will be able to respond to that combination more quickly than two concepts that are not as closely related in their mind - such as "black person" and "friendly".
I urge everyone who reads this to take at least one of these tests for implicit bias. It's not only white/black - there are other racial biases represented as well, such as native american, asian, and arab, as well as tests for gender bias, age bias, sexual orientation bias, and even "body image bias" (fat vs. thin).
A final note: This made me re-think my position on affirmative action. If these biases are so ingrained that they can affect us even when we try not to let them... maybe there is a place in this country for affirmative action. Until we can train ourselves to treat everyone the same, it could be helpful to use this artificial method of integrating our schools and jobs.