This is one of the scarier personnel changes made so far.
For those who may not recall, Bolton is the epitome of chickenhawk. A Vietnam draft-dodger who thinks we should preemptively go to war with basically everyone (including North Korea). He still thinks the invasion of Iraq is a good idea, he opposes the Iran nuclear deal, and once tried to force the intelligence community to say that Cuba was developing chemical weapons.
This guy is incredibly dangerous, and now has the ear of the most incompetent and unstable president we've ever had.