The bubble flare looks like a donut on the end of the brake line. For the MC, the bubble flare on the line seals against a flare inside the MC. So when you tighten down the line, the two seal against each other.
Flare fittings:
http://en.wikipedia.org/wiki/Flare_fitting
Compression fittings:
http://en.wikipedia.org/wiki/Compression_fitting