Millie is the diminutive of several names: Mildred (from Old English mild + þrȳð, meaning "gentle strength"), Camilla (Latin, a young helper in Roman ritual), Emilia (Latin aemulus, "rival"), and Millicent (Germanic, "strong worker").
Millie was very popular in the late 19th century, faded for most of the 20th, and has returned. Today it sits in the U.S. top 400 and is climbing.
Millie 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 Millie reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.