You say the thing is to introduce an explanation, comment, or opinion, that relates to something that has just been said. The thing is is often used to identify a problem relating to what has just been said.
 [spoken] 'What does your market research consist of?'—'Well, the thing is, it depends on ourtarget age group.'
 I've got onto a course. But the thing is, I don't know whether I want to do it anymore.