Tempo.

Tempo is the strength of one player's board relative to the opponent's. If your board is significantly stronger than your opponent's, you have a tempo advantage. Gaining tempo is done by playing minions or removing the opponent's minions. Playing minions is a plus to your tempo, while removing the opponent's is a minus to theirs. The significance of tempo depends on which decks you and your opponent are playing, but it is important to fully understand the concept regardless. 

If you are playing an aggro deck you want to be generating as much tempo as possible until you can use your advantage to reduce your opponent's health to zero. Generally this is done by playing a lot of cheap and efficient minions along with a strong draw effect. The cheap minions generate lots of tempo but since they are cheap, they leave your hand quickly and run you out of gas. For this reason, a strong draw effect is usually necessary in every aggro deck. Imp Warlock, for example, plays a lot of imps which are cheap minions with very strong synergy, and Impending Catastrophe (2 mana Warlock spell that draws a card, plus an additional card for each Imp you control) to refill their hand when necessary. Without the cheap and strong minions, it would be very difficult to generate the necessary tempo advantage to quickly defeat the opponent. 

Control decks want to reach late game to play their big threats and win conditions, so generating tempo in the early game is not as relevant. That doesn't mean that control decks don't care about tempo, however, since the opponent will be generating tempo to threaten the control deck. This is why control decks include a lot of removal, especially Area of Effect (AoE) to efficiently remove large boards. For example, Control Paladin wants to reach late game in order to play Lightforged Cariel (8 mana Paladin Hero that mitigates half the damage you take), The Jailer (10 mana 10/10 that makes your minions immune), Sire Denathrius (10 mana 10/10 instant lifesteal and damage), and any other threats they may include in their deck. As you can see, the cards they want to play are very expensive, but once they are played it is unlikely that they will lose. In order to be able to reach late game to play these expensive but powerful cards, Control Paladin includes Equality (2 mana spell, make all minions 1 hp), City Tax (2 mana spell, deal 1 damage to all enemy minions), and Blademaster Samuro (4 mana 1/6 that deals its attack as damage to all enemy minions the first time it survives damage) to clear the opponent's boards and remove their tempo. By removing the opponent's tempo, you mitigate a lot of potential damage and make it more likely that you can reach late game to play the powerful cards in the deck.

Aggro and control decks are at the two ends of the tempo spectrum. There are other archetypes in between the spectrum such as midrange and combo decks, but how important tempo is in those decks usually depends on the decks themselves. Regardless of the deck, however, tempo will be an important concept in every Hearthstone game, and I hope that this helps you understand it and how to utilize it better to improve your game.