A brand is defined as the supplier of goods or services. Normally, a brand will sell the products or services to another company or individual. Brand Field is also provided in the field mapping section for Add feed only in Wix. 

The Static Value is when you want to apply the same value to all the products in the feed. You can do this by enabling the toggle and entering the Static Value in the field instead of entering the column index or name. 

The field will identify the value as Static Value if you enable the toggle.  

For example:

So, all the products will be imported as BrandA.