The six sense bases (ayatana) are the six pairs of faculty and object through which, on the Buddhist analysis, all experience arises: eye and visible form, ear and sound, nose and odour, tongue and taste, body and tangible object, and mind (mano) with mental objects. Counting mind as a sixth sense with its own proper objects is what distinguishes the scheme from Western enumerations, and it has a consequence: thought is placed on the same footing as seeing and hearing rather than in a supervising position above them.
The analysis is dynamic rather than anatomical. Consciousness arises in dependence on a faculty and an object; the meeting of the three is contact; contact conditions feeling; feeling conditions craving. That sequence sits in the middle of the twelve links of dependent origination, and it is where the practical work is located, because the tradition identifies the junction between feeling and craving as the point at which the process can be interrupted. A very large proportion of Buddhist meditative instruction concerns precisely this junction.
An entire division of the Pali canon's connected discourses, the Salayatana Samyutta, is devoted to the topic, and there the sense bases are called "the all": the Buddha states that the all is just these six pairs, and that anyone who claims an "all" beyond them would be unable to explain it. This is a deliberate restriction of the field of enquiry to experience, and it is among the passages most often cited by those who read early Buddhism as declining metaphysical questions rather than answering them.
The associated practical teaching, guarding the sense doors, is one of the canon's most frequently repeated instructions: on seeing a form, not to grasp at its signs and features, since it is by grasping that unwholesome states flow in. The restraint intended is not sensory deprivation but the interruption of an automatic elaboration, the process the commentaries call papanca, conceptual proliferation, in which a bare sensation is rapidly built into a story with a self at its centre. In Abhidharma the ayatanas form one of three standard classifications of experience, alongside the five aggregates and the eighteen elements, each cutting the same material differently for a different analytical purpose.