@josieskid You can give a post a thumb for any of the reasons already stated, or, if the poster has done something extraordinary, such as an informative, picture-laden post or tutorial, or has posted some information of value to you, you can click on the acorn icon. Be aware though, that while thumbs are simply "likes", and are free, acorns are "NGA currency", in that they're earned, for instance, by posting information, images or corrections to the plant database. Once you have earned acorns, you can bestow them to others, use them to purchase icon badges for your ID header, enter raffles, etc. So, when you click that acorn, you're giving away one from your account. There's a link at the bottom of this page you can follow for more information.