In CSS, the opacity property is used to specify the transparency of an element. The value of the opacity property can range from 0 (fully transparent) to 1 (fully opaque).

Here is an example of how to set the opacity of an element:

.my-element {
opacity: 0.5;
}

This will make the element with the class my-element semi-transparent, with an opacity of 50%.

You can also use the rgba() function to set the opacity of an element's background color. The rgba() function takes four arguments: the red, green, and blue color values (in that order), and the alpha value, which represents the opacity. The alpha value can range from 0 (fully transparent) to 1 (fully opaque).

Here is an example of how to set the opacity of an element's background color using the rgba() function:

.my-element {
background-color: rgba(255, 0, 0, 0.5);
}

This will set the background color of the element with the class my-element to red, with an opacity of 50%.