And how is a Monk guarded as to the doors of the six sense bases?

Here, a Monk, seeing a form with the eye… he hears a sound with the ear… with the nose smells an odor… with the tongue tastes a flavor… with the body touches a physical object… with the mind he recognizes a mental object, does not grasp at the general features or at the details of them.

Since craving and aversion, evil, unwholesome states might flow in upon one who dwells with the sense base of the eye uncontrolled… the sense base of the ear uncontrolled… the sense base of the nose uncontrolled… the sense base of the tongue uncontrolled... the sense base of the body uncontrolled... the sense base of the mind uncontrolled, he applies himself to such control, he sets a guard over the sense base of the eye… the sense base of the ear… the sense base of the nose… the sense base of the tongue… the sense base of the body... the sense base of the mind, attains control of them.

That is how a Monk has the doors of the six sense bases guarded.

(Reference: SN 35.239)

In this Teaching from Gotama Buddha, he shares that one should guard the doors of The Six Sense Bases. This is also referred to as “guarding the doorways to discontentedness”. The practice of Right Mindfulness, to include, The Four Foundations of Mindfulness is how a Practitioner would “guard the doorways to discontentedness”.

All discontentedness experienced in the mind will be experienced through one of these doorways - the eyes, ears, nose, tongue, body, or mind. Through being observant of the mind by practicing Right Mindfulness, a Practitioner is able to guard the mind cutting off and letting go of any arising discontentedness as a bodily sensation ensuring that it does not reach the mind to become discontent feelings.

A Practitioner who trains the mind to be able to control The Six Sense Bases is able to guard those doorways ensuring the mind does not experience discontentedness related to experiences of agreeable or disagreeable forms, sounds, odors, flavors, physical objects, or mental objects entering one of these doorways and affecting the mind.

In practice, this can be employed in multiple ways.