TeX expansion replaces expandable TeX tokens with other tokens according to their current meanings and the scanning context. TeX macros provide replacement lists; some TeX primitives also expand. Subsequent processing can execute commands or build typesetting boxes.[ref]
For example,
\def\firstdemo{AB}\def\seconddemo{\firstdemo} establishes two macros. Expanding \seconddemo first produces the token for \firstdemo; expanding that yields the character tokens A and B. The intermediate result is a token stream. The printed shapes are determined later by computer fonts and layout.Meaning can change between definition and use. With
\def\value{A}\def\callvalue{\value}, redefining \value to expand to B changes the later expansion of \callvalue to B. The stored TeX control sequence still identifies the same name, while its current definition supplies the replacement. This makes the distinction between token identity and current meaning practically useful. New to topics? Read the docs here!