鱼与熊掌不可兼得(yú yǔ xiónɡ zhǎnɡ bù kě jiān dé) literally means : “One cannot get fish and bear's paw at the same time.”
It is a golden sentence from Mencius. Mencius said, "I like fish, and I also like bear's paws. If I cannot have the two together, I will let the fish go, and take the bear's paws. So, I like life, and I also like righteousness. If I cannot keep the two together, I will let life go, and choose righteousness."
Nowadays people use it to express:
You can not have both at the same time.
You must choose one or the other.
You can't have your cake and eat it too.