The Sun has been fairly active lately and studded with sunspots. Here's today's solar image.
Active Region 1461 is that tiny sunspot a bit more than halfway from the center at the 10:00 position (it has rotated away from the limb of the Sun since April 16).
So what is coronal rain? Sunspots are active regions that periodically emit coronal mass ejections (CMEs) of plasma - charged particles over 100,000° F (the details are complicated and the subject of another post). The strong gravitational field of the Sun pulls this plasma back down to the surface but since it's composed of charged particles, they follow magnetic lines of force which form loops about the sunspots. Falling blobs of plasma (many larger than the Earth) fall back into the Sun looking like rain.