From the Accelerant core rules:

"A defense might have a trait associated with it. The verbal should include the defense and the trait with a "by" between them. This can be done to add flavor to an ability, to differentiate two similar skills, or to allow other skills to key off certain defenses."

For example, a game ability might give a player the ability to resist an effect if they invoke the name of a particular god or sphere of influence:

"Resist by Determination!"

The reference to determination is simply to explain what influence, power, or ability the character tapped into to gain that resistance. Many abilities that grant defenses indicate what the character must say during the Verbal to activate the ability. Sometimes you can use these Defensive Traits to gain contextual clues regarding the creature you're facing to help you better know their weaknesses.

For more information regarding Defenses, see the Defenses category.