ConTeXt is a TeX format: an elaborate collection of macros defining a TeX 'dialect', a higher-level typesetting interface that runs on TeX's ca. 300 primitive commands. Next to ConTeXt, other well-known TeX formats are LaTeX and Plain TeX.
A format is precompiled, and loaded by TeX when you run it. ConTeXt ships with its source files, and keeps its compiled format inside the
texmf-cache. In fact, ConTeXt has more than one format: the English, Dutch, etc. interfaces are all separate formats.
$CONTEXTROOT/tex/texmf-context/tex/context/base # source files $CONTEXTROOT/tex/texmf-cache/luatex-cache/context/.../cont-en.fmt # compiled English format
When you alter any of the MkIV source files, you have to manually remake the MkIV format(s) using the
context --make # remake all mkiv formats context --make cont-en # remake the English mkiv format