Make words from the falling letters. Use left mouse button to select the letters. Alternatively, use 'Enter' to submit a word, 'Backspace' to erase a letter, 'Del' to clear a word. Press 'P' to pause.
Try to make words longer than 6 letters, this will keep your multiplier up and the score high. Submitting wrong words or small words will drop parasites (the star shapes), submitting long words (10+ letters) will clear the parasites. When less than 2 letters are left, the game is over.
Also try the 4 challenges: "10 Minutes to Midnight" (score as high as you can in 10 minutes), "Pit Cleaner" (65 letters dropped, try to clear them all), "Ladder" (clear as more words of increasing lengths are dropped), "Alphabet Soup" (find the alphabet as fast as you can).