What does the "+" (plus sign) CSS selector mean?

See adjacent selectors on W3.org.

In this case, the selector means that the style applies only to paragraphs directly following another paragraph.

A plain p selector would apply the style to every paragraph in the page.

This will only work on IE7 or above. In IE6, the style will not be applied to any elements. This also goes for the > combinator, by the way.

See also Microsoft's overview for CSS compatibility in Internet Explorer.

It's the Adjacent sibling selector.

From Splash of Style blog.

To define a CSS adjacent selector, the plus sign is used.

h1+p {color:blue;}

The above CSS code will format the first paragraph after (not inside) any h1 headings as blue.

h1>p selects any p element that is a direct (first generation) child (inside) of an h1 element.

• h1>p matches <h1> <p></p> </h1> (<p> inside <h1>)

h1+p will select the first p element that is a sibling (at the same level of the dom) as an h1 element.

• h1+p matches <h1></h1> <p><p/> (<p> next to/after <h1>)

"+" is the adjacent sibling selector. It will select any p DIRECTLY AFTER a p (not a child or parent though, a sibling).

The + combinator is called the Adjacent sibling combinator / Next-sibling combinator.

For example, the combination of p + p selectors, selects the p elements immediately following the p elements

It can be thought of as a "looking alongside" combination that checks for the immediately following element.

Here is a sample snippet to make things more clear:

body {
  font-family: Tahoma;
  font-size: 12px;
}
p + p {
  margin-left: 10px;
}

<div>
  <p>Header paragraph</p>
  <p>This is a paragraph</p>
  <p>This is another paragraph</p>
  <p>This is yet another paragraph</p>
  <hr>
  <p>Footer paragraph</p>
</div>

Since we are on the same topic, it is worth mentioning another combinator, ~, which is the General sibling combinator / Subsequent-sibling combinator

For example, p ~ p selects all the p which follows the p doesn't matter where it is, but both p should be having the same parent.

Here is what it looks like with the same markup:

body {
  font-family: Tahoma;
  font-size: 12px;
}
p ~ p {
  margin-left: 10px;
}

<div>
  <p>Header paragraph</p>
  <p>This is a paragraph</p>
  <p>This is another paragraph</p>
  <p>This is yet another paragraph</p>
  <hr>
  <p>Footer paragraph</p>
</div>

Notice that the last p is also matched in this sample.

The + selector targets the one element after. On a similar note, the ~ selector targets all the elements after. Here's a diagram, if you're confused:

It would match any element p that's immediately adjacent to an element 'p'. See: http://www.w3.org/TR/CSS2/selector.html

p+p{
//styling the code
}

p+p{
} simply mean find all the adjacent/sibling paragraphs with respect to first paragraph in DOM body.

    <div>
    <input type="text" placeholder="something">
    <p>This is first paragraph</p>
    <button>Button </button>
    <p> This is second paragraph</p>
    <p>This is third paragraph</p>
    </div>

    Styling part 
    <style type="text/css">
        p+p{
            color: red;
            font-weight: bolder;
        }
    </style>

   It will style all sibling paragraph with red color.

final output look like this