A field is simply an attribute defined in your database. Examples of fields:
All records of the same collection share the same fields. For example, each company will have a specific Status (1), Name (2), Headquarter (3), Industry (4), etc.
In the Forest Live demo, we chose to have a limited number of attributes, but this is not mandatory. In fact, you can have as many Fields as you want.
Having a lot of fields only means that you will have to move laterally to visualize your columns. Also, this might impact performance.