A picklist is a list of data values saved to a demographic field. Creating a picklist for demographic fields allows you to ensure data integrity. If ever an import is made with non-matching values, you will be notified to correct the data.
There are multiple variations to flag a gender: M, Male, male etc. If you create a gender picklist, you can ensure all data being stored for your contacts will have the same values, across the board. This will ensure integrity of any segmentation and dynamic content you use in the future.
Utilize the buttons on the right in the picklist section (add, delete, etc.) to manage the list of values stored for the field.
If you need to add a Picklist to your Custom Table fields, please see this article for instructions: How do I setup a picklist for a custom table field?