To start Keyword search does not need any operators. Operators are (Contains), !* (Not Contain),  < (Less than), > (Greater than), etc. The search will look for that value that you enter. In the list view, you are using keywords search if the search says "for text" or "Keyword". If you look at image1, "for text" is a keyword search. It will search any term entered and it will look at anywhere on the form that term exist.

 

If you look at image1 you will see other search options other than "Keyword" or "for text" search. For example, "Number", "Short Description", "Priority", "Caller", etc. These search options are actual fields on the form and columns on the list view. These search options will use operators in their query. For example, if I choose "Number" option type *0000002 in the search bar on the header of a list it will search for a "Number" that contains 0000002. Refer to image3. 

 

Another characteristic of Keyword search is that it is not a column on a list or field on the form. It is merely a search option that looks for terms on a form. If you look at any list view and click on the condition builder (hourglass looking element) and choose keyword as your query option you will see that no operator is an option other than "are". Please refer to image2.