Actually, the first value, when defining two values for background position, is the horizontal one...
As your example stands at the moment, it is invalid - there should be a space between the zero and the minus sign :
As the <a> is 24px high, and the background position is offset 195px upwards, then the <a> will display that portion of the background image between 195px and 215px from the top. The hover will display that portion between 300px and 324px from the top.
background: url(assets/tabs.gif) 0 -195px no-repeat;
I assume the image contains a number of different tab graphics and their associated rollover states (and possibly active states as well).