在官方文档中，给出了这样的定义：

/**
 * \ingroup packetsink
 *
 * \brief Receive and consume traffic generated to an IP address and port
 *
 * This application was written to complement OnOffApplication, but it
 * is more general so a PacketSink name was selected.  Functionally it is
 * important to use in multicast situations, so that reception of the layer-2
 * multicast frames of interest are enabled, but it is also useful for
 * unicast as an example of how you can write something simple to receive
 * packets at the application layer.  Also, if an IP stack generates 
 * ICMP Port Unreachable errors, receiving applications will be needed.
 *
 * The constructor specifies the Address (IP address and port) and the 
 * transport protocol to use.   A virtual Receive () method is installed 
 * as a callback on the receiving socket.  By default, when logging is
 * enabled, it prints out the size of packets and their address.
 * A tracing source to Receive() is also available.
 */

简单来说即为：

这个类继承于Application类，是一个应用层协议。它的作用是接收和消耗被一个IP地址和一个端口号产生的包。