Rashi

And he caused it to touch my mouth... “...and your iniquity shall be removed” This is to cause pain, to atone for your iniquity that you degraded Israel. And his strength was great, for the angel was afraid to take it without tongs, yet he caused it to touch the prophet’s lip, and he [the prophet] was not injured. [This is found] in Tanhuma (ibid.) This is [the meaning] of what Scripture states (Joel 2: 11): “For His camp is very great,” these are the angels; “and stronger” than they “are those who fulfill His word”these are the prophets (Tanhuma ibid.). Jonathan renders: “And in his hand was a glowing coal,” to mean, “And in his mouth was speech.” The expression רִצְפָּה: [means a thing refined in the mouth and with the tongue] (רָצוּף בְּפֶּה). “From upon the altar,” he received the speech from the mouth of the Holy One, blessed be he, from His throne in Heaven, which was directed opposite the altar that was in the Temple.