What kind of check on power does Montesquieu describe in this excerpt?

To prevent this abuse [of power], it is necessary from the very nature of
things that power should be a check to power. A government may be so
constituted, as no man shall be compelled to do things to which the law
does not oblige him, nor forced to abstain from things which the law
permits When the legislative and executive powers are united in the
same person, or in the same body of magistrates, there can be no liberty.
. Again there is no liberty if the judiciary power be not separated.