It's an air inlet tube, but I've never used one in a distillation setup at school ! You'd need a two necked flask, one for the stillhead with condenser and thermometer, and the other for the air inlet.
(I'm assuming a standard distillation setup, not anything more fancy)
Wow You guys are amazing! I`m so sorry I was in a panic at school making sure all the quickfit were complete and it snowed (so I was frozen to the bone) the school had turned off the heating and I don`t get notifications I must check my email junk folder. Basically, you are both fantastic and I`m completely disorganised so sorry and thank you again! Thats a hectic diagram!