You see an advertisement offering work which you can do at home, for example, stuffing envelopes or putting together home assembly kits. You're asked to pay a fee upfront and then find there's no work on offer, you only get paid if you get others to sign up, or you do the work and don’t get paid for it. For example, you assemble a kit and are told the work isn’t up to standard and you won’t be paid.
A genuine home-working scheme won't ask you to pay money upfront and will explain in writing what you are expected to do, how much you will earn, and when you will be paid. You should also be paid at least the national minimum wage.