Radiation can come from only a few select sources, such as the Sapiens, and from radioactive waste barrels. Some ships, like the one above, can spawn as contaminated or can become contaminated when in contact with a radioactive weapon.

Radiation IS VERY DEADLY. You have approximately 3 minutes (or 180 seconds) to get decontaminated. Get to a Commonwealth station, NOT a Corporate station (the Corporate will kick you out), and pay the 250 Credit fine (if you can't pay, you're dead). Another way to get rid of radiation is to have anti-radiation coating applied beforehand, or get a tinker gathering to apply it for you for 1600 Credits (you will be decontaminated as well, it is included in the 1600 Credits). There is also a nifty decon gel that can be applied anytime to the ship that can take away radiation, and it is advised to test barrels (assuming you are near a CW station and have at least 250 Credits) to find these rare life savers.

A nice analogy to radiation was made during a stream of Transcendence by QubeTubers along these lines: “Isn't radiation like a space STD? Just don't get near it and you'll be fine”.
DON'T dock to irradiated wrecks. You WILL regret it. (Unless you have radiation immune armor)