Due diligence is a business term used to describe the process of researching an investment or other business opportunity based on independent sources, to avoid falling victim to a scam, or simply making a poor investment. While something of a term of art in the business world, due diligence is a good idea for pretty much anybody in any field where the acceptance of the unfamiliar is required. (Religions don't like it, preferring intellectual masturbation to independent research on the part of their proselytes and followers.)
Due diligence is often honored more in the breach than the observance, leading to much wringing of hands, gnashing of teeth, and looting of pension funds.
Generally speaking, due diligence involves researching the risks one might be exposed to, learning enough about the field of investment to understand and explore the risks, and hiring professionals when necessary to help analyze the situation. Classic examples would be hiring a professional inspector to examine a house one is considering buying, hiring a lawyer to go over any contract one is considering entering into, or the recommendation to have an independent mechanic you trust examine a (
used pre-owned) car you are interested in.