Hi!
If there is no VLAN specified for a particular SSID, then traffic on that SSID will not be tagged. As a result, if there is only one VLAN on the AP, the traffic goes on that VLAN. If multiple VLAN's are present (i.e. the AP is connected to a trunk port on the switch), then the traffic will be placed on the native VLAN of the trunk.
I hope this helps!