What is sr-only in Bootstrap 3?

Answer

Here is the styling used: (example)

.sr-only {
    position: absolute;
    width:1px;
    height:1px;
    padding:0;
    margin:-1px;
    overflow: hidden;
    clip: rect(0,0,0,0);
    border:0;}

Is it important or can I remove it? Works fine without.

It's important, don't remove it.

You should always consider screen readers for accessibility purposes. Usage of the class will hide the element anyways, therefore you shouldn't see a visual difference.