Appreciations are a way to send genuine kudos to another creative professional. All projects have a blue "Appreciate This" button at the bottom, below the content as well as on the right hand side of the project.
Appreciating a project will add to the project's "appreciation count" and let the member know that you were a fan of their project. One of the ways to sort the Behance gallery is by "Most Appreciated."
If you're logged in when you Appreciate a project, it will be saved and you'll be able to view it later in your "Appreciation Stream," which can be accessed at any time by visiting your Profile Page and clicking "Appreciated" or by visiting behance.net/USERNAME/appreciated. Projects that your network contacts have Appreciated will show up in your Activity Feed.