\documentclass{article}
\newtoks\mytok
\begin{document}
\mytok{a b c}
a b c = \the\mytok
\def\foo{aa}
\mytok{\foo}% NOT expanded YET
\def\foo{bb}
bb = \the\mytok
\def\foo{{aa}}% Must have braces!!!
\mytok=\foo % AND it IS expanded here!!!!
\def\foo{{bb}}
aa = \the\mytok
\newcount\foo\foo=1
\mytok{\global\advance\foo1\relax}
1 = \the\foo;
\the\mytok
2 = \the\foo;
\the\mytok
\the\mytok
4 = \the\foo;
% don't do \the\mytok !!
\mytok{&}
\def\foo{aa}
\mytok\expandafter{\foo\foo}
\def\foo{zz}
aazz = \the\mytok
\end{document}