Why does The Magicians episode have this title?

From what I can tell, the episode is actually titled "The 4-1-1".

I don't watch the show, so I can't speak for the specifics of what the episode was about, but "the 4-1-1" is relatively common slang, at least in North American English.

4-1-1 is the "Directory Assistance" number in USA and Canada. Because of this, the slang term "the 4-1-1" is fairly common in idiomatic usage (for example "What's the 4-1-1?"). See also this question from the EL&U stack exchange.

Giving someone "the 4-1-1" usually refers to providing them with relevant information, or getting them up to speed on what is happening. I do not know the specifics of this episode; but it seems to be a play on that phrase, combined with the fact that it is Season 4 Episode 11.

The phrase Give me the 4-1-1 is used as slang for "Give me the details" or "Tell me what happened".

In context of the show:

In this episode, Julia and the gang finally meet the Binder. As soon as he was released, he revealed The Monster's true motivations as well as the origin of the gods he was murdering and the stones he was collecting.

In essence, he gave them (and us) "the 4-1-1" on the events of the entire season.

It was also a tongue-and-cheek reference at the beginning of the episode when Margo was telling everyone (repeatedly) about her adventures in Fillory.