Bertha is from the Old Germanic beraht (bright). Bertha of Kent (c. 565-c. Her presence prepared the ground for Augustine of Canterbury's 597 mission — she had brought her chaplain and Christian worship to Kent decades earlier. The mother of Christian England.
Featured in Bede's Ecclesiastical History.
Bertha 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 Bertha reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.