I can understand mind is conciousness because mind resolves into conciousness during deep sleep but how to understand body is also conciousness.
Mind does not resolves in deep sleep rather exists in seed form. Had mind merged into consciousness, then samadhi would have ensued. But after waking up, you are still able to recall the experience of deep sleep that you didn't witness a dream. This clearly shows that there was a Seer (Subject) that saw or experienced blankness (object) which is a sign of duality. Hence understand that mind hadn't been absorbed into Consciousness.
Now, how to understand that the body too is consciousness? Just take a simple instance of the ocean and a wave arising on the ocean water. Because of ignorance, we (Wave) limit and identify ourself as a body mind complex entity (a name and form) independent of ocean, failing to realise that underlying essence both Wave and the large expanse of ocean is "Water" itself. In principle, both wave and Ocean is nothing but water. Therefore wave(body) surfacing over the ocean (perceived/unperceived world and beyond) is one Consciousness and has no any separation. The idea of separation arises because of the tendency to identity oneself as a limited name and form entity which is ignorance.
The material body is the annamayakosa. There are 5 kosas or khosas - sheaths which surround the Atman in an individual, the outer material sheath is the annamayakosa. See Where does a Soul attach to the Body? and What are three Sharira and their relation to five Kosha? There are multiple questions on this topic to explain the sheaths more fully.
In his commentary on verse I.17 in Sri Vidyaranya Swami's Pancadasi, Swami Swahananda writes:
...the translation of sarira as 'body' is a bit confusing. It does not mean something with hands, feet, etc. or branches as of trees; but the perishable and perishing outer-coating of the inner consciousness building and guiding it.
The seat of consciousness, the atman, in man is in the buddhi, and although it permeates the body, the outer sheath known as the annamayakosa, it is not the body. When the buddhi withdraws from the annamayakosa, the outer sheath dies. It is because we identify ourselves with the body and think we are the body, that leads us all to delusion rather than identifying ourselves with our true selves, out inner atman. The material body has no consciousness in or of itself.