what is more is an adverb meaning "in addition". It carries one recorded sense.
in addition
“computer chess games are getting cheaper all the time; furthermore, their quality is improving”
“the cellar was dark; moreover, mice nested there”
“what is more, there's no sign of a change”
Definitions, synonyms and semantic relations for what is more come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.