What does it mean when a planet is depleted? If it was in the tutorial ( wall of text ) it seems I've missed it. There is also nothing on the forums ( search depleted ) about it.

Are you talking about a planetary anomaly? Depleted soil? That's a negative planetary anomaly that lowers the food production (and possibly industry, i can't remember for sure) that the planet provides. It's something that you can lower the effect of with certain technology and planetary upgrades.

t's related to the Craver faction trait. You have a FIDS bonus for the 50 first turns after a colonization (Cravers overexploit the natural resources). After 100 turns, the planetary resources are depleted due to the overexploitation (FIDS malus).