Definitions vary, but all writers agree on this: a mental state's content is non-conceptual if an organism can be in that state without having to possess the concepts used in canonically characterizing that state's content.

A more direct definition is this: non-conceptual content is content that is not composed of concepts. But that's too quick, and requires qualifications.

Chances are, those definitions don't make a lot of sense to those who don't already know what non-conceptual content is. I'll try to include some links from here to some writings of my own that explain what contents and concepts are. But that may be a while...