S'il vous plait = polite
S'il te plait = "impolite"
I'm sure you know how "vous" is the polite form when you want to address someone. This is exactly the same as what happens in the "s'il vous plait"-sentence. In English there is no distinction between the polite (you) and impolite (you) form but most other language do have this difference.
Dutch:
u = polite
jij = impolite
German
Sie = polite
du = impolite
Chinese
您 (nin) = polite
你 (ni) = impolite
Hope this helps! Good luck with your French study! :)