Defender +X

From the Comprehensive Rules 4.0:

During the assignment phase, if your assigned companion has the keyword defender +1, you may assign that character to one additional unassigned minion. Defender +2 allows that companion to defend against two additional unassigned minions, and so on. A character with defender +2 (or greater) satisfies any requirement for defender +1.

Frodo and Aragorn face two Uruk-hai. The Free Peoples player could assign Aragorn to one and Frodo to the other. However, this version of Aragorn has defender +1, so he may be assigned to defend against both minions, leaving Frodo unharmed.

When a character with defender +X is given an additional defender +X by a card or special ability, the bonuses are added together.

Further Explanations

Resolving a skirmish: If the total strength of one side is more than the strength of the other side (but less than double), the side with the most strength wins that skirmish. (If there is a tie, the Shadow side wins.) Place one wound on each character on the losing side. When the winning side has one or more characters with the keyword damage +1, then each losing character takes one additional wound for each damage +1.

For Example:

If your Defender +1 wins against 2 Minions, each Minion gains one wound.
If your Defender +1 and Damage +1 wins against 2 Minions, each Minion gains two wounds.
If your Defender +1 loses against 2 Minions, your defender gains one wound.
If your Defender +1 loses against 2 Minions one of which has Damage +1, your defender gains two wounds.
If your Defender +1 loses against 2 Minions each with Damage +1, your defender gains three wounds.

Companions with intrinsic access to Defender bonuses

Cards that Provide Defender bonuses

Dwarven

Disquiet of Our People

Gandalf

Gondor

Elendil's Valor*

Valiant Man of the West

Banner of the White Tree

Ruins of Osgiliath

Andúril, King's Blade

I Will Go

Armor of the Citadel

Elendil's Valor

Horn of Boromir, the Great Horn

Ranger of the North

Rohan

Firefoot (P)

Firefoot

Forth Eorlingas!

No Rest for the Weary

Protecting the Hall

Rohirrim Mount

Firefoot, Mearas of the Mark

Burial Mounds

Spear of the Mark

Sites

Steps of Edoras

Window on the West

Cards that Support companions with Defender bonuses

Gondor

Swordarm of the White Tower

Sites

Flats of Rohan

Cards that Counter Defender Bonuses

Some Shadow (or Site) cards can counter defender bonuses, or at least reduce their usefulness. There's many ways to do that, including but not limited to: preventing more than 1 minion from being assigned to the same companion, triggering harmful effects when a companion is assigned, assigning a minion directly to the defender +X companion to impair the assignment possibilities, assigning directly a minion to a companion which'd be otherwise protected by a defender +X companion, or by simply preventing a defender +X companion from being assigned to a skirmish.