What's the relation between WS and socket.io?

I just installed socket.io for the first time, and at the end it gave me this message:

To install WS with blazing fast native extensions, use
 <npm install ws --ws:native>
 

What does that mean? Is WS replacement for socket.io? I googled around and I can't figure it out. Or is it replacement for node.js? Or is it something I can install alongside socket.io and node.js?

Also I assume the message refers to this ws?

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