If this button is activated, the sounds loaded into drum channels 8 and 9 will be exclusive. In other words, if a sound is played in channel 8 it will be silenced the moment a sound is triggered in channel 9, and vice versa.
The most obvious application for this feature is to "cut off" an open hi-hat with a closed hi-hat, just like a real one does.
When this is activated, the sample playback is calculated using a more advanced interpolation algorithm. This results in better audio quality, especially for drum samples with a lot of high frequency content.
High Quality Interpolation uses more computer power - if you don't need it, it's a good idea to turn it off!
Listen to the drum sounds in a context and determine whether you think this setting makes any difference.