The letter "K" is a substitute for 1,000. So "150K" means 150,000. It usually refers to money. People use "K" this way in spoken English, because it's easier to say than "thousand". Here are some other examples:

40K ("forty K") = 40,000

3.5K ("three point five K") = 3,500

930K ("nine hundred and thirty K") = 930,000

You can use numbers from one to 999 with "K". If a number is higher than that, you should count in millions.