Legends are usually very old stories with little or no evidence to prove them. Legends are often passed on by word-of-mouth. They are very similar to myths.
Legends can also be famous or historically significant people, places, art, etc. We sometimes say of someone who is extremely famous that they are a "legend" or "of legendary fame".
Examples of legends are Ali Baba, the Fountain of Youth, and the Loch Ness Monster. Some legends are stories about real people; others are not. Robin Hood for example might have been real but most the stories about him are definitely fiction.
For some places, legends are the earliest history. For example, the origin of ancient Rome is known only from legends, as are the earliest dynasties in the history of China.