The CSS selector Specifies the first child elements of the element

TR td:nth-child (2) {    background-color:Gray;}

is the second TD attribute of TD in Tr.

tr:nth-child (2n+0) {    background-color:#F0F0F0;}

This is a multiple of TR's 2.

Use the formula (an + b). Description: Indicates the length of the period, N is the counter (starting at 0), and B is the offset value.

Here, we specify the background color of all p elements that have a multiple of 3 in the subscript:

p:nth-child(3n+0){background: #ff0000;}

The CSS selector Specifies the first child elements of the element