The offset works like a blank column that will stay before your column. For example, if you want a column that will have half of the size of the screen and will be exactly in the middle, you will have to do:
<div class="col-sm-6 col-sm-offset-3"></div>
A full row has 12 columns. This way you will have 6 columns (half of the row) and a offset of 3 columns. It will be exactly in the middle of the screen.
Take a look on Bootstrap documentation.