k is kilo and refers to 1,000. 500k are 500,000. This can have a lot of use cases, sometimes one wants to sell 500k Bitcoin (unlikely) and sometimes users refer to exchange rates in Satoshi: 500k Satoshi basicly mean you can buy something at the rate of 0.005 Bitcoin (0.00500000).

2Bytes are two units of digital information. It tells you how much space is required to store two characters like OK on a disk or in memory. 2 Bytes can be broken down into 16 Bit of plain binary information. I'm not sure in which context this could be related to anything Bitcoin-specific. Maybe you add an example reference.