Equip is a keyword ability found on artifacts with the subtype "equipment". By paying the equip cost, you can attach the artifact to a creature, who is referred to in the text as the "equipped creature".
Equipping is played as a sorcery. When an equipped creature leaves play, unlike local enchantments, the equipment remains in play.
The first cards with equip appeared in Mirrodin. It was so flavorful and so popular, that it went evergreen instantaneously. Ninth Edition was the first core set to feature the keyword. 
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 702.6. Equip
- 702.6a Equip is an activated ability of Equipment cards. “Equip [cost]” means “[Cost]: Attach this permanent 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 a permanent has multiple instances of equip, any of its equip abilities may be activated.
From the glossary of the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- A keyword ability that lets a player attach an Equipment to a creature he or she controls. See rule 301, “Artifacts,” and rule 702.6, “Equip.”
- Whispersilk Cloak -
Artifact — Equipment
Equipped creature is unblockable.
Equipped creature has shroud. (It can't be the target of spells or abilities.)
Equip (: Attach to target creature you control. Equip only as a sorcery.)