Flying

Flying is a static ability that makes a creature unblockable except by creatures with flying or reach.

Comprehensive Rules
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 702.17, “Reach.”)

702.9c Multiple instances of flying on the same creature are redundant.