Legends are special cards who have either been picked up in packs, earned in events or elevated from normal playing cards at the end of each season. Each card is marked with a year corresponding to the real world year of their career; there can be multiple Legend cards of the same player with different years (and different stats).
Flashback journeys in the Faction page are only available to Legends, with highly lucrative one-time rewards. Additionally, each Legend can be used in certain regular modes to bolster your regular teams, not as players but as mentors with unique effects.
Season 1 of Mr Football will not include matches between Legends, but that is planned for future seasons when enough cards to form a team are available. For the purposes of playing in matches, Legends are assumed to have 100% fitness at all times and do not need to be upgraded further. Star ratings also do not apply to them, since they are in a separate category with generally higher attributes. Multiple copies of a Legend card with the same year do not confer extra benefits, and may be exchanged for faction points.