What's "deep down" mean and when's it used?

Native speaker’s answer
Rebecca
"Deep down [inside]" refers to the part of your mind or heart where your strongest, most secret feelings are. It's often used when you want to express that you recognize or acknowledge something is true, even if it is difficult or hard to accept. Ex: I act like I'm confident but deep down, I feel insecure all the time. Ex: Deep down inside, everyone just wants to be appreciated and loved.