A mixed drink is a soft drink to which alcohol (usually liquor) has been added, to give it an alcoholic content and change the flavouring. Many people make mixed drinks because they dislike the taste of straight liquor. There were very few mixed drinks in the modern style in period, as anyone who could afford liquor would drink it straight up.
Certain mixed drinks are more or less SCA-specific, including Tranya, Caidan Blue and Swamp Frog. All three involve a commerical powdered soft drink, such as Kool-Aid or Freshie, to which large amounts of liquor (typically vodka) have been added. In all three cases, the beverage is typically served in a clear glass or plastic pitcher with a glowstick floating inside, producing an eerie diffuse glow.
Note: If you are ever served anything glowing at an SCA event, consider yourself warned.