If you're not from the US then you can ignore it. If you are from the US then you can still ignore it if you want to, but they're giving US authors the option to charge and receive sales tax, when they sell to a buyer that is in the same state as them.

The tax will be added to the cost of the item, paid by the buyer, given to the author, and then the author will have to give that money to the IRS or whichever government department handles sales tax.

Slightly worrying that authors might charge sales tax and then not report it justbtobgetvthe extra money. Not paying taxes is one thing, but charging other people taxes and then not paying on those taxes is another thing entirely... the IRS can be pretty ruthless from what I hear!

Apparently the rules relating to sales tax, and the percentage, and whether you should be charging it at all, varies depending on the state in question. It could be argued that Envato should be handling the sales tax if they're selling the items, but they're now going to be giving authors the option to handle sales tax if they should be and if they want to.

That's pretty much it in a nutshell.