Felix Felicis is liquid luck, which makes the person who drinks it lucky for a certain period of time. It is the colour of molten gold, and large drops leap like goldfish from its surface, never spilling.
Felix Felicis is a desperately tricky potion to make, and disastrous to get wrong. When brewed correctly, the drinker finds that all their endeavours tend to succeed until the effects wear off. When taken in excess, the potion can cause giddiness, recklessness and dangerous overconfidence. It is highly toxic in large quantities.
One tiny bottle of Felix Felicis is enough for twelve hours' luck. Professor Slughorn offers a bottle of this quantity as a prize in his Potions lesson to the student who creates the best Draught of Living Death during the lesson.