Ya'Seem was a site of major archaeological interest.
Its discovery by M'Tell was considered one of the greatest archaeological discoveries ever, comparable to Schliemann finding Troy. (TNG: "The Chase")
It was not specified what exactly Ya'Seem was, although the mention of M'Tell first stepping on it suggests it was something to be landed on.
According to the script
, the pronunciation for Ya'Seem was "ya-SEEM".