Bolster Rule 701.39 Glossary

A keyword action that puts +1/+1 counters on the weakest creature a player controls. See rule 701.39, “Bolster.”
Flying Rule 702.9 Glossary

A keyword ability that restricts how a creature may be blocked. See rule 702.9, “Flying.”

Rulings (2)

February 25, 2015 Wizards of the Coast
You determine which creature to put counters on as the spell or ability that instructs you to bolster resolves. That could be the creature with the bolster ability, if it’s still under your control and has the least toughness.
February 25, 2015 Wizards of the Coast
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 Aven Tactician’s bolster ability.