Stress Facilitates learning Fear Associations but impairs Fear

What this idea basically boils down are the two terms - 'Fear Associations' and 'Fear Extinction'.

Fear Association is the process taken by parts of our brain to associate an action with a fear that it induces in us.

Fear Extinction is the phenomenon of ending the fear association when the action or object is no longer fearful.

Now, coming to the sentence, stress helps us identify different actions or things to be afraid of but it disables us to withdraw the fear when the action or object is no longer to be feared.