What exactly is offset in bootstrap?

Offsets are used for spacing elements in the responsive grid.

The unit is based on the column layout.

You can define an offset this way : col-[breakpoint]-offset-[number of colums]

In this example, admitting our layout is made of 12 columns :

<div class="row">
    <div class="col-md-6 col-md-offset-3">
        <p>test</p>
    </div>
</div>

Means that in the medium range of the grid system, the element will have a width of 6 colums and there will be 3 blank columns before the element (and as a consequence, will have 3 blank colums after).

The result of this is a div of 6 colums width, centered in the container.

There is a example showing how it renders in the Bootstrap documentation. https://getbootstrap.com/docs/4.3/layout/grid/#offsetting-columns

Offset means: Move columns to the right using the .col-md-offset-* classes. These classes increase the left margin of a column by * columns.

For example col-md-offset-3 increases the left margin by 3 for medium devices.