Krenko is a 2RR creature with a 3/3 statline, and an ability as simple as it is dangerous: tapping Krenko creates X 1/1 red Goblin creature tokens, where X is the number of Goblins that the player controls
It can help you replace some bad cards in your hand or discard a card you want in the graveyard
For example, if you choose to counter target spell, return target nonland permanent to its owner's hand, and have target player draw a card, that permanent will be on the battlefield while that spell is countered (and may modify, affect, or trigger on that event) but will not be on the battlefield while that player draws a card
Mostly though, the power lies in the ability to return a card from your graveyard to your hand when they get turned face up
It spawned entire control-based archetypes the entire time this set was in Standard and made control decks dominant for many months