Agni - the essence of will
Agni refers to fire - or metabolism. Everything that enters the body or mind must be metabolised in order to be processed and eventually in one way or another - internalised. Consider how when food goes in it either becomes a part of us or gets rejected and eliminated. It’s the same with food as it is the senses, but food is physical and gross, thoughts and impressions are much more subtle and are internalised in a more subtle way too.
But whatever is taken ‘in’ must be met by Agni - our inner fire. When Agni is strong and metabolism is complete both body and mind keep healthy. But when Agni is weak and metabolism left incomplete there’s a type of undigested toxic residue that forms - known as Ama.