A node represents another entity in the wireless network. It's usually a base station when operating in BSS mode, but can also represent entities in an ad hoc network.
A node is described by a struct ieee80211_node, declared in <sys/net80211/ieee80211_node.h>. This structure includes the node unicast encryption key, current transmit power, the negotiated rate set, and various statistics.
A list of all the nodes seen by a certain device is kept in the struct ieee80211com instance in the field ic_sta and can be manipulated with the helper functions provided in sys/net80211/ieee80211_node.c. The functions include, for example, methods to scan for nodes, iterate through the node list, and functionality for maintaining the network structure.