The people believed that when the water was disturbed, it indicated the presence of an angel who would heal the first to enter the pool after the disturbance (“which may have been caused by the intermittent bubbling up of a natural spring” [Morris 302]).

3b-4 (KJV) waiting for the moving of the water.  For an angel went down at a certain season into the pool, and troubled the water: whosoever then first after the troubling of the water stepped in was made whole of whatsoever disease he had.

“The end of verse 3 and all of verse 4, as given in the KJV, were probably added as a marginal explanatory gloss early in the second century, and from thence gradually became incorporated in the text. John’s failure to mention that the pool was thought to have medicinal qualities tempted transcribers to add a few marginal words in the nature of comments.” (McGarvey TFG 195)