I have this against you, that you forgive Jezebel—What Jesus has against the whole church is their acceptance of the false teacher. This teacher is causing great sin to occur in the fellowship, but the fellowship just accepts her without correction. Jesus is not pleased with this at all.
I gave her time—Jesus, in the past, already warned the false prophet, the leader of the sect, to stop the false teaching, but the teacher didn’t listen to Jesus, so Jesus is going to punish her.
Throw her to a couch—The prophet and (her) group of followers will be made seriously sick until they repent. This is a common thing for God to do against those in serious sin (see Psalm 32)
Repent of their deeds—Jesus wants them to stop their actions.
I will kill her children—Some of the followers will die of the sickness.
Perhaps this seems severe on Jesus' part. Okay, it absolutely seems severe on Jesus' part. But we need to see the circumstance from Jesus' perspective.
What this sect is doing is forsaking the kingdom of Jesus, the only way that one might obtain eternal life. Not only are they forsaking this way of life, but they are giving strength to the enemy that persecutes the way of life. In the long run, Jezebel is killing people, in the most permanent way possible. Jesus has given her a chance to stop killing people, but she refuses. Jesus isn't taking any more chances that she might kill more people in his kingdom. He is taking serious action-- making her and her followers sick and some of them will die. This will show them all the seriousness of her sin.
Interestingly enough, the original Jezebel also killed people when it served her purposes.