Deck control is the concept of being able to play all of your cards in your deck every turn. Decks with high deck control are typically very strong as they can gain payload cards and be guaranteed to play them every turn. However gaining deck control is an investment and will decrease as soon as more stop cards are added to it (e.g. victory cards)
Deck control can be achieved by having a high density of draw cards and/or having a small number of cards overall, especially stop cards.
Note that this does not necessarily mean you have to draw every card in your deck. For example filtering cards like Border Guard, Embassy or Forum can be used to put playable cards in your hand and leave unplayable cards (eg Curse and Estate) discarded.