What effect does clicking on my believers have?

Whenever I click on one of my believers, a number in green (which so far has always been 100) pops up above them and fades away. This doesn't appear to affect any of the other numbers or progress bars on the screen so I'm wondering what exactly is happening here?

Regardless of the condition of the believer (ie: they could be collapsed on the ground seemingly about to die) they will suddenly jump up and start doing stuff.

What effect does clicking on my believers have?

Clicking on a follower rejuvenates them! Or, more to the point, heals them completely (unless playing against an AI).

Your followers will weaken over time during exploration. Sadly, they will eventually die. If you see a follower sitting down, click on them to restore their health! That'll keep them going for a while. They will, over time, begin to walk slower, sit down, and die. Being seated means that they are quite near death.

It's worth mentioning that in matches against an AI opponent, your heals only count for 40, as opposed to 100.