The service set identifier — what the network calls itself.
Reading a capture
A zero-length SSID is not a bug: it is how an AP hides its name, and how a station asks "who is out there?" in a wildcard probe. The name is octets, not text, so nothing stops it containing bytes that do not render.
Subfields
Element ID — Identifies which element this is. Zero here.
Length — The length of everything after this octet, in octets.