In computer networking, a service set is a set consisting of all the devices associated with a consumer or enterprise IEEE 802.11 wireless local area network (WLAN). The service set can be local, independent, extended or mesh.
Service sets have an associated identifier, the Service Set Identifier (SSID), which consists of 32 octets that frequently contains a human readable identifier of the network.
In layman's terms, when referring to a wireless networks SSID we are talking about the name of the network that appears when a wireless router or access point broadcasts an available wireless connection.
For a detailed explanation please see the wiki.