questions on css nth-child code example

Example 1: child css

p:nth-child(2)
 	{     
 		background: red;
 	}

Example 2: css nth child

:nth-child(1) { /*advantage is you can do it for 2nd, 3rd etc. */
	/* styles go here*/  
}

Example 3: css nth child

:nth-child(3) { //the number is the child number you are targeting
	//styles here 
}