It shows the available space where text can be expanded.

You can see this dashed area after text which means it is area where text can be expanded

*, html, body {
    box-sizing: border-box;
    margin: 0;
}

div {
    position: relative;
    background-color: lightgreen;
}

button {
  display: flex;
  width: 100px;
}

<div>
    <button>1</button>
</div>

It is white space. For example you have a container with 100% of width and two divs inside, one of those with 50% and another width 40% of width, it means that there is 10% of space empty... this 10% would be shown in this purple dashed line area by the Google inspector.