What are these lines that I frequently see on doppler radar?
If they propagate outward from thunderstorms, they are outflow boundaries. Cold air flows down from the thunderstorm with precipitation and spreads outward upon hitting the ground. This air moves ahead of the storm and forces warmer air upward, suspending clouds and other particles. That's what is visible on radar.
More: https://en.wikipedia.org/wiki/Outflow_boundary