GeoIndex - BGS

This is mostly just meant as a post to let others be aware of this. It's a system much like that of Google maps etc, that allows overlays of scientific data over the UK. Fault lines, bedrock, deposits, magnetic readings, gravity and so on, can all be viewed with this tool in your browser.

All rather interesting, but one thing I'm not certain of my understanding for and will therefore ask other users here, is what exactly magnetic data is indicative of?
It seems to follow some relatively uneven patterns over much of the UK, but I wonder why it does this, when there is a rather large amount of (presumably more magnetic) area around the Great Glen.

So can anyone here educate me a bit on this? Gravity readings make sense enough, less dense rock, more dense etc, igneous and sedimentary rock, etc, but I would like to know what magnetic info shows.