I am confused with the W8BEN form, can somebody explain details?

I know that I have to accomplish the W8BEN form to receive payment, but i'm a bit confused about it's purpose and I don't want to do anything that I'm not sure of. I read about the W8BEN form and it's purpose is to tax Foreign workers of income derived from the US. I became confused about the W8BEN when I was accomplishing my tax info. and all that was stated is that I agree to be tax exempt as I am not a US citizen. Any clarifications would be helpful thank you!