What are the white hearts?

White hearts (which the wiki calls Eternal Hearts) sit above regular hearts, but below soul hearts in the order of what takes damage first. When you fill up a white heart or you go down a level with half a white heart, it turns into a heart container.

According to the mechanics guide, any regular heart drop has a 1/50 chance of becoming an eternal heart. This excludes hearts from the Bloody Penny or Little C.H.A.D., which are always half hearts. In addition, in the super secret room that contains a single eternal heart, any other regular heart drop in that room automatically becomes an eternal heart.