Meaning of S. V. P.

I am currently travelling through France and here and there again and again, I notice an abbreviation written on quite every corner that I can't really interpret: SVP!

I have seen this abbreviation (usually written as s. v. p.) in all sorts of places, really everywhere, regardless of location and situation. On mailboxes, on doors, in the toilet, in shops, at train stations, inside and outside, usually in combination with some request.

But I still can't figure out what this "secret code" could mean. Can someone explain it to me?