802.11 networks come in two main flavors. "Managed" or "infrastructure" networks (that Kismac identifies as <managed>) involve one or more dedicated "access points". In an "ad hoc" network (Kismac: <ad hoc>), individual computers communicate directly, without any access point involved.
In a managed network, an access point sends "beacon frame" broadcasts, typically every 100ms (i.e., 10 times/second) containing data that identify the properties of that particular 802.11 network (e.g., whether WEP is required, etc.). In an "ad hoc" networks with no access points, one of the individual stations (usually the first one that has ad hoc mode turned on) takes on the chore of sending the beacon frames instead.
By default, the SSID is included in those beacon frame broadcasts. If it is, then Kismac can easily and quickly determine the SSID.
Many models of access points have an option to "hide" the SSID, which simply means that the AP won't include the SSID in the beacon frames it sends. However, the SSID will still show up in some other types of 802.11 frames that are sent less often (e.g., "association request", "reassociation request", "probe response", and probably a few other types of frames). In that case (particularly on an essentially idle network), it can take quite a while for Kismac to detect the SSID when listening passively.
Kismac may show <hidden ssid>, <no ssid> or blank when it hasn't yet detected an SSID. I'm not precisely sure when each of those is shown (and mileage has varied between different versions of Kismac).
A "probe request frame" is sent by computers trying to find access points. Asking for access points with a specific SSID (or via "broadcast SSID" to try to get a response from any access point within range). Access points send "probe response frames" back. (However, hidden access points will generally not respond to probes to "broadcast SSID".) The Kismac <probe> shows probes.