A prefix is a word, letter, or number placed before another, as in centi before metre, or kilo befor gram.  As you probably realise, a good proportion of the questions in a physics exam contain calculations involving very large or very small numbers. Think of the size of a galaxy or the size of an atom. To describe very lage or very small numbers, scientests will often use prefixes to the units. Think millimetre or Gigawatt. It is essential that you know , understand and can reacall the main prefixes. Kilo Mega Giga and Tera, are for larger numbers abd are 10 to the power 3,6,9 and 12 ( the 3 times table). Milli micro nano pica are 10 to the power minus3,minus 6,minus9 and minus12 ( the minus-three times table ).