What is the function of CSS selector plus sign + and css selector plus sign?

What is the function of CSS selector plus sign + and css selector plus sign?

What is the function of the CSS selector plus sign +:
This selector can match the sibling element next to the specified element.
The code example is as follows:

<! DOCTYPE html> 
The above Code sets the font color of the first li element next to the li element whose class property value is first to red.
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