When you look at a badminton court, you will see lots of different lines painted on. The court is usually marked up for both singles and doubles, which use different court sizes. The very outside lines form the court for doubles games. In a doubles rally, the shuttle is allowed to land anywhere on the court.
The singles court is somehwat smaller, with the next lines in from the outside edge making up the outer edge of the single court. When next to the doubles lines, they are often referred to as Tramlines, because they resemble tram or train tracks!
An easy way to remember is that tramlines are in for doubles, out for singles.