'Equip' was introduced during the introduction of 'Artifact - Equipment'

702.6. Equip

• 702.6a. Equip is an activated ability of Equipment cards. "Equip [cost]" means "[Cost]: Attach this Equipment to target creature you control. Activate this ability only any time you could cast a sorcery."
• 702.6b. For more information about Equipment, see rule 301, "Artifacts."
• 702.6c. If an Equipment has multiple instances of equip, any of its equip abilities may be used.

Translation:

• 702.6a. It's an activated ability which, when activated, is used to move the equipment between permanents. You can only do this when you could cast a sorcery.
• 702.6b. Further reading - 301, "Artifacts."
• 702.6c. If an artifact has both Equip 1 and Equip 2, then you can pay 1 or 2 to activate the ability.