I agree with Jeff, both that it should go and that it probably came from the Mathematical SE community (originally MO, possibly by way of maths-SX).
On MO, it means "I'm tagging this 'soft-question' because I know it shouldn't really be there but hopefully by admitting that then Andrew Stacey won't immediately vote to close it.". Of course, it actually has the opposite effect since anything tagged 'soft-question' on MO almost surely shouldn't be there and I almost automatically vote to close such questions!
To be clear: I wouldn't vote to close the questions here that are tagged 'soft-question' (those that aren't already closed), but I agree that the tag 'soft-question' adds absolutely nothing to those questions.
If you can figure out a cure, put it into a retrovirus and I'll see if I can infect MathOverflow with it.