“I can definitely relate to that.”

Your friend is telling you that he's bored at work and unsatisfied with his job. You feel the same way about your job. You say:

I can definitely relate to that.

(someone) can relate to (a feeling)

Being able to "relate to" a feeling or experience means that you understand it. It might mean that you've had the same feeling or experience before. In other words, this is an expression for showing sympathy.

Most parents can relate to the feeling of being overwhelmed when you have your first child.

I've been made fun of my whole life. I don't know if that's something that you can relate to.

definitely (do something)

In the example above, the speaker says "I can definitely relate to that" to show that he really, completely agrees with his friend.