you don't understand it well."
I've got a huge file of potential story ideas just waiting for me to write them. Some ideas are just a fragment--a character or a situation that seems interesting. Others come to me with ready-written log lines like (if I was Stephanie Meyer): "New girl falls hopelessly in love with beautiful boy who is really a vampire."
I have to believe that the fully-formed log lines would probably be easiest to write. They have a plot built in to them. Certainly you could pitch such a story to an agent/editor with ease.
I get most frustrated with my writing when I've lost direction. I'm not sure what emotion a character should be feeling or how a scene should unfold. During those times, I don't think I can explain simply what my story is all about.
That's when it would help to have that log-line to keep me on track. Stay focused.
Happy writing, all.