If you find that your die-cuts are not completely cut out or your embossing does not have enough definition, then try adding a piece of paper or cardstock to your sandwich. A shim usually lays on your bottom Standard Cutting Pad, under your die or embossing folder.
The necessity of shims will vary among users. Due to the tolerable variances in the measurements of all products associated with cutting, results can slightly differ depending on the material being cut, the die being used, or even from one Big Shot machine to another. You should experiment to identify what works best (shim, no shim, extra shim, etc.) for the material you are cutting and the die you are using