Enchant

Enchant is a keyword ability that restricts what an Aura can target and be attached to. Up until Ninth Edition of the Core game, Auras were know as "local enchantments". Local enchantments carried all the rules in their type, which agonized R&D. Now that the rules are in the actual Rules Box, Auras can be more diverse and it's easier for beginners to recognize that Auras are Enchantments.

From the Comprehensive Rules:

• 502.45. Enchant
  • 502.45a Enchant is a static ability, written “Enchant [permanent or player].” The enchant ability restricts what an Aura spell can target and what an Aura can enchant.
  • 502.45b For more information on Auras, see rule 212.4, “Enchantments.”
  • 502.45c If an Aura has multiple instances of enchant, all of them apply. The Aura’s target must follow the restrictions from all the instances of enchant. The Aura can enchant only permanents or players that match all of its enchant abilities.
  • 502.45d Auras with the “enchant player” ability can target and be attached to players. Such Auras can’t target permanents and can’t be attached to permanents. Rules 212.4d–i apply to an Aura with enchant player in relation to players as they normally would for permanents.

For your reference [CR 212.4]:

• 212.4. Enchantments
  • 212.4a A player may play an enchantment card from his or her hand during a main phase of his or her turn, when he or she has priority and the stack is empty. Playing an enchantment as a spell uses the stack. (See rule 409, “Playing Spells and Activated Abilities.”)
  • 212.4b When an enchantment spell resolves, its controller puts it into play under his or her control.
  • 212.4c Enchantment subtypes are always a single word and are listed after a long dash: “Enchantment — Shrine.” Each word after the dash is a separate subtype. Enchantment subtypes are also called enchantment types. Enchantments may have multiple subtypes.
  • 212.4d Some enchantments have the subtype “Aura.” An Aura comes into play attached to a permanent or player. What an Aura can be attached to is restricted by its enchant keyword ability (see rule 502.45, “Enchant”).
  • 212.4e An Aura spell requires a target, which is restricted by its enchant ability. Other restrictions can limit what a permanent can be enchanted by. If an Aura is coming into play by any other means than being played and the effect putting it into play doesn't specify what it will enchant, the player putting it into play chooses a permanent for it to enchant as it comes into play. The player must choose a legal permanent according to the Aura’s enchant ability. If an Aura is coming into play from the stack and there is no legal permanent for it to enchant, the Aura is put into its owner’s graveyard instead of coming into play. If an Aura is coming into play from any zone other than the stack and there is no legal permanent for it to enchant, the Aura remains in the zone from which it attempted to move instead of coming into play. The same rule applies to moving an Aura from one permanent to another: The permanent to which the Aura is to be moved must be able to be enchanted by it. If it can’t, the Aura doesn’t move.
  • 212.4f If an Aura is enchanting an illegal permanent, or the permanent it was attached to no longer exists, the Aura is put into its owner’s graveyard. (This is a state-based effect. See rule 420.)
  • 212.4g An Aura can’t enchant itself, and an Aura that’s also a creature can’t enchant a permanent. If this occurs somehow, the Aura is put into its owner’s graveyard. (This is a state-based effect. See rule 420.)
  • 212.4h The permanent an Aura is attached to is called enchanted. The Aura is attached to, or “enchants,” that permanent.
  • 212.4i An Aura’s controller is separate from the enchanted permanent’s controller; the two need not be the same. Changing control of the permanent doesn’t change control of the Aura, and vice versa. Only the Aura’s controller can play its abilities. However, if the Aura adds an ability to the enchanted permanent (with “gains” or “has”), the enchanted permanent’s controller is the only one who can play that ability.