A check List is a list of possible BDSM activities, which is then completed by a submissive to indicate which activities they have an interest in. This gives the Dominant an idea of the submissive's experience to date and of their desires and limitations. There are two key parts to the list therefore, Experience and Desire/Limitations. The submissive is then able to clearly indicate to the Dominant what they would like to do (and at what level of preference) and that which they have already done. Check Lists are usually completed at the start of a Dom/sub relationship and may even influence the descision to proceed with the relationship at all.
The list of possible BDSM activities is wide and varying, and an example is shown here.