This is the 1273rd most frequent Turkish word.
bilen
"One who knows."
Here, 'bilen' is used as a participle to describe 'an individual who knows' within the context of relative clauses.
Ahmet her sorunun cevabını bilen bir öğrenci.
Ahmet is a student who knows every answer.
In this case, 'bilen' is acting as a stand-alone noun referring to 'a knowing person'.
Böyle bir sıkıntıyı ancak bilen anlar.
Only someone who knows such a problem can understand.
Here, 'bilen' is used attributively to describe an expert with knowledge about the specific subject.