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

The + symbol in CSS is known as the Adjacent Sibling Selector. It is used to select an element that immediately follows another element and shares the same parent element.

For example, consider the following HTML code:

html
<div>
    <h1>Title</h1>
    <p>This is a paragraph.</p>
    <p>This is another paragraph.</p>
    <span>This is a span element.</span>
</div>

If we use the following CSS code:

css
h1 + p {
    color: red;
}

This selector will select all <p> elements that immediately follow the <h1> tag and set their text color to red. Therefore, in the above HTML example, the first <p> ("This is a paragraph.") will be selected and its color will change to red, while the second <p> tag will not be affected because it is not immediately following the <h1> tag.

The Adjacent Sibling Selector is very useful, especially when you want to style elements in a specific order. It makes CSS rules more precise and targeted, helping to build more complex and finely controlled layouts and styles.