What happens to brain during meditation?
I'm not sure about what happens in the brain while someone is actually meditating (i.e., the state effect of meditation), but a meta-analysis from last year (Fox et al., 2014) looked carefully at what long term brain changes result from meditation. This is from the abstract:


  Anatomical likelihood estimation (ALE) meta-analysis found eight brain regions consistently altered in meditators, including areas key to meta-awareness (frontopolar cortex/BA 10), exteroceptive and interoceptive body awareness (sensory cortices and insula), memory consolidation and reconsolidation (hippocampus), self and emotion regulation (anterior and mid cingulate; orbitofrontal cortex), and intra- and interhemispheric communication (superior longitudinal fasciculus; corpus callosum).


The paper is available online here: http://www.christofflab.ca/pdfs/Fox2014-MeditationMorphometry.pdf