When Hera learned that Leto was pregnant with Zeus' children, she banned Leto from giving birth on the mainland, or any island. Leto found the floating "island" of Delos, where she was able to bear her children.
Leto's role in the Greek myths is fairly limited. She is known for the story of Apollo and Artemis' birth, but is barely mentioned afterwards.
In Roman Mythology, Leto is known as Latona.