A phrase used by neds (chavs if your english) all over the world. They say it when they don't understand something or sometimes when they do but just want attention from the teacher/parent/authority figure present. It really means:
"Can you explain how that works" But being neds they just knock words of of sentences to let them speak more rubbish faster.
Teacher:"Why doesnt helium react?"
Parent: "Because it is a noble gas"
Authority Figure: "It has No outer electrons"
Ned: "How That!?!?!?!"
Chav: "Aye! How That?"