What does the title mean and have to do with the story "The Chaser"?
The word "chaser" has multiple meanings, and interestingly enough, each of them can be related to the story in different ways. The most obvious meaning of the word is "one who pursues". This can refer to both the young man who goes to the old man who deals with potions, and the young woman whose affections he is trying to win. The young man is pursuing the woman, and after he uses the love potion on her, it is she who will be pursuing him.
A "chaser" is also a drink which is taken after a first, stronger one, ostensibly to take the taste of the first away. This "chaser" can be related to the second potion the old man sells, the poison that takes away the effect of the love potion he sells first. The love potion, which sells for only a dollar, will cause the woman to fall in love with the young man, in a way that is so total and consuming that he will soon wish he had never set eyes upon her. He will need the second potion, the poison which sells for five-thousand dollars for a single teaspoonful, as a "chaser" to rid himself of the bad experience, or taste, engendered by the first.