Purge tower or waste tower is a additional printed structure for holding the waste that happens when changing the filament. There's going to be some wasted material when changing the filament because it's impossible to pull all material from the nozzle, because some of it is molten. So the old material needs to be purge to somewhere. In slicers this can be a separate purge tower or similar structure. Here's example with the purge tower on the left and the actual model on the right: https://www.dropbox.com/s/d9xsh3mj6p...15.04.jpg?dl=0. Yeah, the amount of waste can be quite large...
Prusa MMU is a similar system to mine and it uses Prusa Slic3r to do the purge tower: https://www.youtube.com/watch?v=MnOWX4Ujtv8. The newer MMU2 is more advanced, but it still uses same multiple in - one out hotend setup and purge towers.