Magnets have 'poles' - so like a north and south pole. Likewise, MRI scans do too; they work by flipping the polarity of the magnets rapidly and that 'flipping' means that the water molecules inside of you react to the magnetic field flipping and so an image is produced.

MRI gives a huge amount of information and can sometimes be unhelpful as it may show problems that are unrelated to why you have come in and, as a result, you may end up with new symptoms!