jroy,
As the name implies 'infill' is whatever material you use to occupy the spaces in the walls between the posts and girts or other horizontal members. In the olden days (still practiced some today in replications) this material could be daubing (a mixture of clay and straw with or without woven small sticks for lath) or brick carefully fitted into the spaces.
If you use dimensional lumber, batt or sprayed insulation and wallboard you 'infill' with these modern materials. All theses methods are in lieu of SIPs or other externally applied 'walls'.
There are many books on the subject.
deralte