Short answers, pulled from the story.
Hesiod wrote in his Theogony around 700 BC that Mnemosyne was the daughter of Sky and Earth. Diodorus Siculus later confirmed this parentage in his Library of History during the 1st century BC.
Zeus disguised himself as a mortal shepherd to approach Mnemosyne for nine consecutive nights according to Hesiod's Theogony. Their union produced nine goddesses known collectively as the Muses who governed various arts and sciences.
Ancient poets invoked Mnemosyne during the opening lines of oral epics like the Iliad and Odyssey to accurately remember and perform long poems they were about to recite. Memory served as an essential building block of civilization within Greek oral culture.
Initiates in Orphism learned instead to drink from the Mnemosyne which was the river of memory. This act stopped the transmigration of the soul and preserved personal history across lifetimes unlike drinking from the river Lethe.
Statues of Mnemosyne appeared in sanctuaries dedicated to other gods throughout Ancient Greece including the shrine of Dionysos at Athens and the Temple of Athena Alea. She was worshipped specifically in Lebadeia within Boeotia where she played an important part in the oracular sanctuary of Trophonios.