'Flanking' is a combat mechanic which has to do with the idea that this creature makes the blocking creature worse. It only applies when a creature is attacking and is blocked, otherwise it remains dormant.

702.22. Flanking

• 702.22a. Flanking is a triggered ability that triggers during the declare blockers step. (See rule 509, "Declare Blockers Step.") "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
• 702.22b. If a creature has multiple instances of flanking, each triggers separately.

Translation:

• 702.22a - Flanking happens as soon as your opponent declares blocker(s) for the creatures with flanking.
• 702.22b - Flanking does care if a creature has multiple instances of it, as each will trigger.