A perfect example of newspeak, commonly found in the US federal government and frequently found in the same sentence as 'diversity'. Excellence is an intangible noun derived in recent decades from an adjective meaning anything that is not directly in opposition to a mission statement. Basically it is supposed to mean 'good-ness', but better than good, really good. But without any increase in quality of work to back it up. So it can be anything from poor to mediocre to decent to great to outstanding.
In all concrete reality, excellence means nothing, but you'll see it everywhere to describe everything.
"She has demonstrated outstanding achievement in the field of excellence while serving her country during the difficult circumstances administering corrections at Abu Ghraib prison"
"At the Library of Congress we value diversity and excellence, while fostering a compassionate environment of professionals"