Sense doors are your senses: Seeing; Hearing; Taste; Touch; Smell & Mind

To protect is to shield or guard against. Example: If seeing is protected, then it is not impacted. This could be as easy as you don’t look, thus protected. It can also mean that you don’t react and so you are unfazed. It is not the absence of stimuli, it is the absence of reaction (not to be confused with indifference or desensitization). If these reaction opportunities (sensory triggers of information) are met with consistent neutrality, then you are protected.

Eventually you develop a default mindfulness muscle, of various calibrations, towards stimuli. Day to day has a setting and deep meditation has a setting. Senses are in various states of protection or can be.

The mind can be an originator of information without the other 5 senses triggering it, thus it is a sense. So too, it should be met with mindful neutrality and equanimity so as to achieve slavery liberation.

To strengthen this introspect/insight empirically you should meditate. The “Depth” course is a simple, non-dogmatic, vehicle to do your own science experiments for the validity of this information.