The term Maggid Shiur literarly translates from Hebrew to mean "sayer of Shiur".
This term is used to refer to the Rabbi that lectures in a Yeshiva

Yeshiva

Yeshiva is a Jewish educational institution that focuses on the study of traditional religious texts, primarily the Talmud and Torah study. Study is usually done through daily shiurim and in study pairs called chavrutas...

 or Kollel

Kollel

A kollel is an institute for full-time, advanced study of the Talmud and rabbinic literature. Like a yeshiva, a kollel features shiurim and learning sedarim ; unlike a yeshiva, the student body of a kollel are all married men...

.
A Maggid Shiur is generally a Rabbi

Rabbi

In Judaism, a rabbi is a teacher of Torah. This title derives from the Hebrew word רבי , meaning "My Master" , which is the way a student would address a master of Torah...

 who lectures on advanced and in-depth Talmudic

Talmud

The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

studies.

The stature of a Maggid Shiur is generally a much sought after position by the typical Yeshiva student.
The position of Maggid Shiur is usually obtained by those well versed in the Talmud.