The rivnut itself contains a thread and screws onto the mandrel, it is then inserted into a predrilled hole.
By squeezing the handles of the tool together, the mandrel is pulled back into the tool forcing the rivnut to deform and so griping the panel it has been inserted into. Then simply unscrew the mandrel from the rivnut using the knurled knob and you're left with secure place into which a bolt can be fastened.
Jacknuts are very similar but designed for weaker or brittle materials and perfect for GRP. When they deform they spread "fingers" to exert there grip over a much wider area. It is possible to use the same tool to insert a jacknut but care is required as a rivnut tool is designed to exert much more force than is needed for a jacknut. There is a similar but specific tool for jacknuts but it isn't up to the job of inserting a rivnut. Rather than buy both, I'll just have to learn to be careful !