Overflow scroll in CSS is used when you want to have a scrollbar for a certain element, but you want the scrollbar to appear all the time even when it is not necessarily needed. It allows the content to be scrollable both vertically and horizontally.
Here is an example of how to use overflow scroll in CSS:
```
.example {
width: 200px;
height: 200px;
overflow: scroll;
}
```
In this CSS declaration, any content inside an HTML element with the class `example` that exceeds the specified width and height (200px in this case) will be scrollable.
But if you want the scrollbar to appear only when needed (when the content is larger than its container), you can use `overflow: auto;` instead.
You can also specify overflow properties specifically for each direction: `overflow-x` (horizontal) and `overflow-y` (vertical) with values of `scroll`, `auto`, `hidden`, or `visible`.
For example:
```
.example {
width: 200px;
height: 200px;
overflow-x: scroll; /* Horizontal Scroll */
overflow-y: hidden; /* No Vertical Scroll */
}
```
This will make the element scrollable only horizontally and no scrollbar for the vertical axis.
Just a reminder, in some cases, scrollbars might not appear on devices that don’t need them, like touchscreen mobile devices.
