Who is described in verse 14? It is obvious that the description is connected to the vision of the Son of Man in Daniel 7. Given that the vision concerns the exaltation of Jesus, it means that we should regard the one like the son of man here as Jesus as he will look on the Day of Judgement. We can see from the description that he will be very glorious.
First, he is seated on a throne of glory (white cloud); second, he is wearing the crown of glory; and, third, he has the instrument of judgement in his hand (sickle). The description is of him waiting to act, waiting for the moment which the Father has put in his own authority, the day of the second coming of Jesus. Here is Jesus gathering his people to himself. It only takes him a moment to do so.
In verse 18, in a second description of the harvest, an angel comes from the altar and tells another angel to deal with those whom he will throw into the place of judgement. The altar here is probably the altar of incense, and here we have a reminder that in some ways the Day of Judgement is connected to the process by how God answers the prayers of his people for vindication.
Again, the description of the judgement indicates that it is very severe. Such an amount of blood is very difficult to imagine. The point that John is being shown is how awful the judgement will be. This image could be taken from Isaiah 63 where the Messiah is said to crush his enemies as if in a great winepress. Where is the winepress? It is outside the city of God. What a terrible location to be in, experiencing the judgement of God.