This is a Main Menu command - it should be typed when in the game's main menu (similar to 'developermodeon'), not into the developer console. Typing this into the main menu will enable Regrow Mode, which makes it so that when your character sleeps, 10% of all cut down trees regrow fully. Type regrowmode into the main menu again to disable this mode.
The syntax for the regrowmode command is as follows:
Search our database of 109 The Forest commands...To the Commands