Even after you've completed your GCSEs, you will need to measure things! The metric system is internationally recognised, and very simple in use.
The sizes of distances and objects are measured in metres, multiples of metres (e.g. km), and fractions of metres (e.g. cm).
1,000 m = 1 km
The k and c letters are prefixes. The k stands for kilo - meaning 1,000 - so a km is a kilometre or 1,000 m.
The c stands for centi - meaning 1/100 - so a cm is a centimetre or 0·01 m.