What is the meaning for saith?

Answer 1

"Saith" is an obsolete way of saying "says"
As in ...
"Vengeance is mine, saith the Lord", which means "Vengeance is mine, says the Lord." It can only be used where you would use "says", never where you would use "say".

Answer 2

"Saith" is an archaic third-person singular form of the verb "say." It is rarely used in modern English and is often found in older texts or literature.