'K' is short for 1,000. It comes from 'kilo'.

K is the scientific abbreviation for "1,000" from Kilo, and the military slang "Klik" meaning a kilometer. In the super-short text message world, "k" is much shorter than "1,000," so it is now a common abbreviation. ;-)

It stands for "kilo". It is a metric prefix to indicate a multiple. Look at: https://en.wikipedia.org/wiki/Metric_prefix