"Don't be afraid, I feel it too". Feel what?

Hi everyone..

I tried to do a research about this but found nothing, so.. sorry if it has been already discussed....

During the interrogation scene, Kylo tells Rey: "Don't be afraid, I feel it too".

What does Rey feel, that Kylo feels too? Does he mean in fact: "don't be afraid, cause this fear I feel"? Or does he feel her ability with the Force? or that they have a bond?

But in this case, why is he so surprised that she resists him and even manages to read his mind? I had the impression that Kylo realized Rey was strong with the Force onlywhen she claims he's afraid of not being as strong as Vader, but maybe I got it wrong...?

Thank you for your ideas...