Ok try brushing it on, leave it for a minute, then lightly drag the tip of the brush over the surface to break the bubbles, any drag marks should dissappear (assuming oil based). They usually occur when air is trapped and expands from heat, or using a roller, or if you were over enthusiastic with the stirring beforehand and made the varnish frothy. Rubbish brushes wont help either, i like Hamilton perfection for varnishing.
All the above refers to an oil based varnish, if you’re using water based crap then your on yer own ;-)