stands for Miniature Magnetic Plasma Propulsion.
It's probably the absolute coolest and most elegant deep space propulsion device ever suggested.
How it works is as follows.
First a large magnetic field is generated, ideally using superconducting coils; it's a few feet across.
Then a plasma is injected into the magnetic field. This sets up a current through the magnetic field, which is trapped in the coils, the plasma goes around and around the coils, but due to mutual repulsion the plasma grows, and grows, grows and grows.
Once it reaches 20km(!) across it has a huge cross-sectional area relative to the solar wind- in fact the solar wind striking it at 350km/s to 800km/s imparts quite a bit of thrust on it.
This tugs on the magnetic field of the space craft and causes the space craft to accelerate directly away from the sun.
It has been calculated that speeds as high as 90 km/s may be achieveable. (That's easily fast enough to leave the solar system entirely and never return), should you wish to do that.
However there are difficulties. For one, the plasma is at millions of degrees and this tends to melt the spacecraft. For another the propulsion system has never been tested for real, any number of issues may crop up, although it has been successfully tried in the laboratory in vacuum chambers, but only with a plasma a few feet across.
Thirdly, the drive doesn't change the tangental speed much- to go from one planet to another in the solar system mainly requires that the speed around the sun change; this drive mainly pushes straight away from the sun, which leaves the tangental speed unaffected.
Still, the basic physics behind it is sound, the earths magnetosphere works this way, and although the earth is too heavy to go anywhere because of this effect, the physics behind it all is atleast somewhat well understood.
Interestingly the idea came out of research into Bussard Ramjet; where calculations showed that the magnetic field for collecting hydrogen from interstellar space gave too much drag. Instead this device makes use of the drag, and doesn't require nuclear fusion for it to work.