Apala (अपाला) means "unprotected" or "without protection" in Sanskrit. Apala Atreyi was the Vedic poet-sage — author of Rig Veda 8.91, her hymn to Indra, who in return for her devotion healed her of a skin ailment that had caused her abandonment by her husband.
Featured in Rig Veda 8.91.
Apala 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.
In Pythagorean numerology the letters of Apala reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.