Bolster

Bolster is a keyword action introduced in Fate Reforged where it replaced Outlast from Khans of Tarkir as the mechanic associated with the Abzan Houses. Bolster is also featured in Dragons of Tarkir, associated with Clan Dromoka.

Description
When Bolster N, you choose a creature with the least toughness or tied with the least toughness among creatures you control and put N +1/+1 counters on it.

Rulings

 * Bolster itself doesn't target any creature, though some spells and abilities that bolster may have other effects that target creatures. For example, you could put counters on a creature with protection from white with Abzan Skycaptain's bolster ability.
 * You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves.