Kayagni is a Sanskrit word, and maybe it has not so uniquely meaning, I will try to explain agni first. Kaya-Agni, Agni means the fire element (one of five) in the human physiology, however it is also present on the outside, one of the elements in every creation.
The agni is responsible for metabolism and activities, if we do not have agni – we do not exist. The agni, or fire we find in us, is in form of Pitta (constitution or body-type).
When Agni is balanced (healthy): The food is nicely digested, absorbed, assimilated and transformed into our body system. The waste products and toxins are eliminated from the body by the natural way. We feel happiness, contentment, bliss, joy, and fulfillment. We are healthy and nourished, good strength, immunity, nice complexion etc.
To understand the word “Kaya” it carries a meaning in different contexts:
Deha (Human body), Kayagni (Fire in the body), Manas (Mind), Hridaya (Heart), Sakala Shareera (Full body).
Kayagni can be seen as Jatharagni, which is located in duodenum and part of small intestine, is the fire in this area. However, if we see Kayagni as fire in the body, it sums up all the 13 types of Agni’s including Jatharagni. In many circumstances Kayagni and Jatharagni are the same and being used interchangeably.