'Flying' is what is called an 'evasion' ability, it determines who can and cannot block a creature in combat. It was part of Magic from the beginning.

702.9. Flying

• 702.9a. Flying is an evasion ability.
• 702.9b. A creature with flying can't be blocked except by creatures with flying and/or reach. A creature with flying can block a creature with or without flying. (See rule 509, "Declare Blockers Step," and rule 702.15, "Reach.")
• 702.9c. Multiple instances of flying on the same creature are redundant.

Translation:

• 702.9a-b. Flying only allows other creatures with Flying or Reach to block this creature.
• 702.9c. Flying is redundant if there are multiple instances of it.