A baal teshuvah, "one who has returned", (pl. baalei teshuvah) is person who has returned to God.
This may be either a person who sinned and then returned to God through the process of repentance or a tzaddik who never sinned, because teshuvah can refer to the general process of reconnecting the soul to its source in God.
Most often the term refers to a person who has sinned and repented, and in this sense, our sages teach (Berachot 34b) that "baalei teshuvah. stand on a level where even complete tzaddikim cannot stand.”
The reason is that a baa1 teshuvah has succeeded in elevating aspects of the world and of his own soul that were on a very low level, whereas a tzaddik is never directly involved with such low things.
In addition, a baal teshuvah, having been far away, feels a greater thirst for God than a tzaddik.
Also, a baal teshuvah must transcend his own nature and reveal new powers within himself, while a tzaddik serves God with powers that were always available to him.
–Rabbi Adin Steinsaltz
In Learning from the Tanya by Rabbi Adin Steinsaltz