A shot sheet is a full log of all the shots you want to include in your film - essentially, it is a checklist filled with minute details that will give your film a sense of direction and efficiency.
Below are a couple of videos that will describe what a shot sheet is, how it is used in pre-production, and why it is necessary when filming. In addition to these videos, there is a JPEG file containing a basic shot sheet template that you can copy and use for your pre-production planning.