Datums, described in more detail here, are locations on a part that indicate a "base" from which measurements are taken. You can specify datums for your design but are not required to do so.
To specify datums:
- Draw a straight line.
- Apply menu Line | Machine | Datum.
- Use Comments To Machinist text to label each datum (usually A, B, C).
- Generally, 2D parts have 2 datums and 3D parts have 3.