A de-tinned tip is one which cannot wet with solder. This exposes the plating to oxidation and degrades the heat transfer efficiency of the tip.
The de-tinning is caused by:
1) Failure to keep the tip covered with fresh solder while not in use.
2) High tip temperatures.
3) Insufficient melting in soldering operations.
4) Wiping the tip on dirty or dry sponges and rags (Always use a clean, wet, industrial grade, sulfur-free sponge).
5) Impurities in the solder, iron plating, or on the surfaces to be soldered.