Hearthstone is a digital collectible card game where players take turns reducing the opposing hero's health to win. Each turn a player can play up to seven cards from their hand, including hero-specific abilities and minions. The game has five different modes: Arena, constructed mode for drafting permanent decks, Tavern Brawl that changes weekly, Practice mode for honing skills versus AI and single player challenges Curse of Naxxramas and Blackrock Mountain.
The typical time limit for each turn in Hearthstone is 75 seconds, with animations that can extend turns a little bit. Occasionally though you'll run into an opponent who will take their turn much longer than normal. Some of these extended turns are due to a lack of understanding of how long an action takes (which is understandable) but others have more to do with the player simply taking too long thinking about their moves.
Taking too long for your turn can have disastrous consequences. Most obviously it can allow the opposing player to build up a massive amount of lethal damage by queueing up their actions over the course of your turn. You can prevent this by checking for potential lethal damage before pressing end turn. This has to do with game knowledge and knowing if your opponent has a lot of cards in their hand that could kill you.
Then there are the cases where your opponents simply clog up their turn with too many actions or because they're playing a card that requires a significant amount of setup like a giant creature. These issues have been ongoing for quite some time now, even after Blizzard tweaked the length of each turn.