Like many words, "yet" has several definitions.
It can mean "but" or "even though". It is generally used when expressing the idea that thing 1 is true, but thing 2 is also true. "Be honest yet witty" means "be honest, but at the same time be witty".
It can be used to refer to an event which as not taken place as of a specified time. It is generally used when you expect the event to happen eventually, or when you are still waiting for the event. "He hasn't arrived yet" means that, as of this moment, he hasn't arrived, but it implies that we do expect him to arrive sooner or later. If you don't expect him to arrive at all or you've given up waiting, you would leave off the "yet" and simply say "He hasn't arrived."
It can mean "even more" in phrases like "yet another" or "yet more". This is usually used as an intensifier. "Yet another mathematical problem" means you've already seen another, and now here is one more. It is typically used when you'd expect the quantity already seen to be enough. Depending on context getting "yet another" might be good or bad. Like, "Oh, yet another rude jerk to deal with", versus, "Oh, yet another delicious candy bar!"