The phrase "letted be" is an archaic form of "let it be," which means to allow something to happen or to leave something as it is. The spelling "letted" is pronounced as /lɛtɪd/ using IPA phonetic transcription, with emphasis on the first syllable. The -ed suffix in letted indicates past tense, so it could be interpreted as "allowed it to be." This phrase is generally not used in modern English and has been replaced with the more commonly used phrase "let it be."
The phrase letted be is derived from Old English. The word let initially meant to hinder or prevent, but its meaning shifted over time to to allow or permit. The verb be simply refers to existence or state of being. Therefore, the phrase letted be essentially means to allow to remain or let it be.