Difference between zero: false and required: true in rails

In rail travel, what is the difference between null: false

and required: true

?

For example, if I write:

t.string      :phone_number,      required: true

      

        
        
        
      

    

this is the same as:

t.string      :phone_number,      null: false

      

        
        
        
      

    

If they are different, how are they different from each other?

Thank you in advance:)