It means “rejoicing with the Torah”, and alludes to the celebration that takes place every year when the Jews read the ending section of Deuteronomy and begin to read the book of Genesis once again.
From 11th century BCE, the 9th day of Sukkot took its name and ritual from what nowadays we know as Simchat Torah. Its festive nature is obvious; we dance with the Torah on the synagogue, and the other one is turn around seven times.
Those who are not carrying the Torah may flap colourful flags, showing their ecstasy and contributing with their effervescence to this joyful moment.