This is the 10777th most frequent German word.
ruefer
"Rufer" in German means "caller" or "crier."
In this case, 'Rufer' is used metaphorically to describe people whose messages or warnings are ignored, akin to 'crying out in the wilderness'.
Die Rufer in der Wüste werden oft nicht erhört.
The callers in the desert are often not heard.
Here, 'rufer' is actually a misspelling of the verb 'rief' or 'ruft' (calls out); correcting this, 'ruft' would be the appropriate verb form for the subject rather than treating 'rufer' as a noun.
Er rufer laut nach Hilfe.
He calls out loudly for help.
Here, 'Rufer' is a noun referring to someone who calls or shouts, specifically in the context of an individual who vocalizes from a distance.