(In LaTeX2e everything in the formula is set in a bold font.)
The \unboldmath declaration switches back to the normal math italic font.
Note, these commands should not be used in math mode. Thus the scope of the declaration must include the math mode expressions to which you want it to apply.
If you want only part of a formula to be bold an \mbox command is useful:
\( Regular formula +
\mbox{\boldmath$ Bold part$} = 0 \)
Both \boldmath and \unboldmath commands are fragile.