CSS property to pad text inside of div

The CSS property you are looking for is padding.
The problem with padding is that it adds to the width of the original element, so if you have a div with a width of 300px, and add 10px of padding to it, the width will now be 320px (10px on the left and 10px on the right).

To prevent this you can add box-sizing: border-box; to the div, this makes it maintain the designated width, even if you add padding.
So your CSS would look like this:

div {
    box-sizing: border-box;
    padding: 10px;
}

you can read more about box-sizing and it’s overall browser support here:

https://www.paulirish.com/2012/box-sizing-border-box-ftw/