An example of a travelling show is a circus. In such travelling shows, there were frequently folks who made "potions" or 'medicines" said to cure all kinds of diseases. (Generally, a glass of water would have better effect.) An expression used in the United States for such 'potions' or "cure-alls" is "snake oil".
"potions" is an old way of saying magical medicines. Typically, witches brew up potions to make people fall in love, and things like that. Potions sort of implies that they don't really work, or do anything real. Potions are usually a drink that makes something amazing happen in a story - like when Alice in Wonderland drank a potion and it made her 10 feet tall. They are fantasy drinks.