Emerson is an English surname meaning "son of Emery" — Emery from the Old Germanic Amalric, meaning "work-ruler." The name became associated with the American philosopher Ralph Waldo Emerson (1803-1882), one of the founders of Transcendentalism.
Emerson as a girl's first name is recent — it entered the U.S. top 1000 in 2003 and has risen rapidly. Today it sits in the U.S. top 100 and is used about 70% for girls.
Emerson does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
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In Pythagorean numerology the letters of Emerson reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.