Conversation so far:
Who said  Kenneth, what is the frequency?
William Tager.
Who is william tager?
the man who attacked Mr.
Rather.
Why did he attach Dan Rather?
Tager claimed he thought television networks were beaming signals into his brain.
Why did he say kenneth, what is the frequency?
to find out the frequency the networks were using to attack him, so that he could block it.
Was Dan Rather hurt?
punched from behind.
Did William Tager serve time for the attack?
Tager was paroled in October 2010.
why did he think tvs were beaming signal into his brain.?
CANNOTANSWER.
How long did it take to find tager after the attack?
In 1997, a TV critic writing in the New York Daily News solved the mystery.
Is there any other relevance to kenneth what is the frequency?
What's the frequency, Kenneth?" became a popular-culture reference over the years.

Question: What is the reference?
Self-contained rewrite: What is the popular-culture reference of what's the frequency Kenneth?