A blow-off or blow off tube is a type of high flow airlock where a tube and bung\seal are placed at the high point of a fermenter and led to a reservoir where the end of the tube is placed beneath a sanitizing solution. This allows for the expulsion of co2 and excess fermentation foam\material.
A blow-off is a great airlock to use when you have minimal head space available in your fermenter and or if you are concerned about a rapid or high foam fermentation. If you ever walk out to your fermenting beer bubbling out of a traditional two or three part airlock, a blow-off would be better alternative to use in that situation, as it allows for a greater amount of carbon dioxide and foam transmission while still maintaining a sanitary environment. A blow-off is rarely necessary in a secondary fermentation unless you are providing additional fermentable sugars at that point.