That's to write a post where you comunicate the fact that you're accepting comissions, which mean that you're willing to make works for other people if they pay you. I'm currently looking for someone to hire me because I need money but apparently I'm not that good
sekru
Basically, the "open for commissions"-button below the user description is just a button that you can use to link to your commission price-sheet if you are open for commissions.
If you aren't, there's no reason to worry about it.