What are the rules for placing 1/1 counters in Magic: The Gathering (MTG)?

In Magic: The Gathering, 1/1 counters can be placed on creatures as a result of certain card effects or abilities. These counters increase the creature's power and toughness by 1 each. Players can place 1/1 counters on creatures during their main phase when they have priority, following the rules and restrictions specified on the cards that allow for the placement of these counters.