A thin Font is a single-line font. A person can put a pen, crayon, or a number of tools right into the "claw" of a cutting machine rather than a blade and use the machine to "draw" instead of "cut". When you do that with a regular font (.ttf or .otf) the machine traces the outline of the font so rather than having a single line, you get a double line.
When you use a font (.ttf or .otf file) on your computer, it will typically look something like this example (unless it is an outlined version of a font):
When you use a font (.ttf or .otf file) in the software of your cutting machine, it will trace the outside perimeter of each letter and typically look something like this example):
Thin fonts are created so that a cut machine does not trace the font but makes them with single strokes much like you would if you were writing.
See instructions on how to install and use your thin font.