Voice can be a difficult concept to explain. Essentially it is when people speak in a unique and distinctive way.
Everyone has their own mannerisms, attitudes, and body language. These are the attributes that make human beings different from one another.
In the Charlie Brown comic Peanuts, Linus always carried around his blanket, sucked his thumb and had his own unique ways of expressing himself. Some people call these things "tags" and they are an extension of voice.
Nathan Bransford said it well: "The other thing I'm noticing about dialogue is that it is most effective when it is very clear it is the voice of a very particular character or characters saying the words, rather than words that could be said by anyone. Everyone has their own way of speaking in real life, but with dialogue on the page it seems even more important to counter expectations, to avoid cliches, and to make it sound original." You can read his entire post here.
Even if characters are using the exact same words, no two people would ever say anything in precisely the same way.
Watch this short clip to see exactly what I mean.
I hope this helps explain voice. If you have any comments or suggestions to add, please click on the comments section and add your thoughts about voice.