[wooger]

Can someone explain what the lines mean?

[robhawkes]

The squiggly ones across the country? Those are the National Energy System Operator (NESO) transmission boundaries for Great Britain. They represent areas of the country that are tracked for the purposes of managing constraints, which is when the transmission network over the boundary can't handle the amount of power flowing over it. When they are overloaded (or rather, before that happens) NESO will step in and "balance" the various zones by turning up or down generators.

[arethuza]

I note that one of those lines seems to take a big detour up and round Cruachan?

https://www.visitcruachan.co.uk/