This term means different things to both cultures, due to Elven immortality. Among the Elves it seems to be a sign of commitment that is not necessarily permanent; it signals that Elves have chosen to live together as long-term partners with a mind to stay together, and presumably raise children. However, they do not demand permanence. The words of a pledge indicate that they may later decide not to.
Among the Chakka, it seems to be a more typical form of marriage.