Jamieson, Fausset and Brown on Revelation 14:15

15. Thrust in--Greek, "Send." The angel does not command the "Son of man" (Re 14:14), but is the mere messenger announcing to the Son the will of God the Father, in whose hands are kept the times and the seasons.

thy sickle--alluding to Mr 4:29, where also it is "sendeth the sickle." The Son sends His sickle-bearing angel to reap the righteous when fully ripe.

harvest--the harvest crop. By the harvest-reaping the elect righteous are gathered out; by the vintage the Antichristian offenders are removed out of the earth, the scene of Christ's coming kingdom. The Son of man Himself, with a golden crown, is introduced in the harvest-gathering of the elect, a mere angel in the vintage (Re 14:18-20).

is ripe--literally, "is dried." Ripe for glory.