'Chuck' is an informal word for 'throw', so 'chuck a left' means make a quick left turn when you're driving. This is an Australian expression, but it would certainly be understood in the UK.
A tarn is a mountain lake, and moss is a kind of thick, green, spongy vegetation. A fringe is an edge. So a moss-fringed tarn is a lake that has moss growing round its edges. There's nothing specifically Australian about these words - they're standard English and you could find them in any dictionary.