Rainfall intensity. The sharp borders are from discontinuities in the colour scale.

Probably rainfall, just light rain where I am currently but expect to be under the heaviest part in around 2-3 hours. If I am reading the met office chart correctly up to 16mm/hour of rain. Probably nothing to be overly concerned with unless you are outside at the time.

Density of precipitation methinks, red yellow green blue from high to low

Edit: actually that makes no sense. Maybe it's height of rain cover with the above colours, and the black spots indicate density?

Shit, I actually don't know.

Looks fearsome though

Edit2: Best I can find, but it's not that parseable

https://openweathermap.org/map_legend