Chancellor is the head of the government in some European countries including Germany, Austria and Switzerland. The title "Chancellor" means the same thing as "Prime Minister". For example, Italy has a Prime Minister while Germany has a Chancellor but they have roughly the same job.
Chancellor is also a title given to public officials in some other countries. For example, Daijō-daijin or Chancellor of the Realm was a government position in Japan in the late Nara and Heian periods.