Rainbows must reach the mold to make rainbow candies. To make the rainbow rapids reach the mold, you must clear all the blockers blocking the path. Blockers (except for crystal candies and candy bombs) can stop the flow of the rainbow rapids.
Once the rainbow rapids have reached the mold, the mold will fill up and produce a rainbow candy, which is then automatically collected. The rainbow rapids then disappear, along with its corresponding cannon and path.
Currently, the rainbow rapid mold cannot be encased in any encasing blockers. Assuming that it could be encased, the mold would need to be unlocked before the rainbow could fill it up.
The rainbow rapid's mold has the tilemap code 156.
This is the second destructible blocker that can't be cleared through regular means. It can only be cleared by the rainbow rapids reaching it.
Up until 1.182, the rainbow rapid's mold could have candies underneath it that would be revealed when it was removed. After 1.182 this no longer applies; rainbow rapid's molds no longer hide candies underneath them. However, jelly can be hidden underneath rainbow rapid's molds, as seen in level 8427.
The blocker is likely based on real-life jelly molds.