If you're a user of the search engines, you know about the minus operator to exclude terms from your results.
Example: To find salsa recipes that do NOT have tomatoes in them,
[ salsa recipe -tomatoes ]
That does just what you think, returning results that do NOT have the word "tomatoes" on the page.
So.... what does + do?
Most people think it means the opposite of minus, but, alas, is doesn't. (Although I agree that would make sense!)
This what + means: DO NOT change the search term in any way. No synonyms, no stems, no nothing. Search for exactly this word. (In other words, it's very much like double-quote for a single word.)
Example: My Mom told me yesterday that she was going to hear some "authentic joiker music" at the local Scandanavian outlet. She was excited about it, and I naturally asked "so... what's joiker?"
A quick Google search for joiker is pretty useless--all of the results are about JOKER as the word gets spell-corrected to something I didn't really want.
This is when you want to use +
[ +joiker ]
And that tells you pretty quickly that it's a traditional Sami (aka Lapplander) style of singing. It's pretty interesting, actually... Wikipedia tells us (with a spell-correction to yoik) that a joik is a song that tries to "transfer the essence" of a person or place to the listener, rather than being "about" a person or place.
In other words, + is the same as the Bing command noalter: (example, on Bing: [ noalter:joiker ] )
Double quotes (on both Google and Bing) serve to turn off synonymization for strings of words. Example:
[ "joik music" ]