A copper once explained to me that if it's a solid white line you have to come to a standstill before proceeding.

Dotted line means to "give way" ("yield" in US), and you can keep rolling so long as it's clear.

Obviously most people keep rolling even if there's a solid line but if you get pulled over, that would be the reason why.

It's the same at junctions / intersections in the UK, at least.