A tonne is a unit of weight equal to 1,000 kilograms — the same thing Taiwanese students call 一公噸. It is the metric unit, used almost everywhere in the world for heavy things: cargo on a ship, rice harvests, rubbish collected in a year, the weight of a whale, carbon dioxide put into the air. Here is the part worth being careful about. English also has a word 'ton', spelled without the -ne, and it is a different amount. In Britain a ton is about 1,016 kg; in the United States a 'ton' is about 907 kg. So the three words sit close together but do not match. To avoid this mess, scientists and international writing often say 'metric ton' or write the symbol 't'. American writing mostly does not use the spelling 'tonne' at all, so if you are reading an American article, 'ton' there probably means the American 907 kg one. In casual speech there is also a loose use meaning simply 'a huge amount', but people almost always spell that one 'ton' or 'tons': 'I've got tons of homework.' You would not write 'tonnes of homework' — the -ne spelling stays with real measuring.