For example, if you click the Share button in Safari, then AirDrop, you can send the current page to any nearby Macs. You can do this from most apps that feature OS X’s Share button, such as Ember and Day One, and your recipient’s Mac will automatically open the item you sent in the relevant app.
Both Macs need to be fairly recent to support AirDrop and within about 30 feet of each other. While WiFi needs to be on, they don’t have to be connected to the same WiFi network to find each other and exchange files. You can learn more about AirDrop for Mac and Macs that support it at Apple’s support site.