The short answer: a meal with around 0.3 g of protein per kilo of body weight, plus carbohydrates to match your session, within the next one to two hours. Everything else, from the shake in the changing room to the anabolic window, is fine-tuning. This article gives you the numbers: how much, when, which foods, and what differs for strength training, running and losing weight.
What is post workout?
Post workout simply means "after training". A post-workout meal is therefore the first meal afterwards, whether that's a shake, a skyr bowl or dinner. The counterpart is the pre-workout meal, the meal before exercise, which gives you energy for the session. These terms come from the fitness scene; your body doesn't know them. After every workout it deals with the same thing: protein to repair the muscle fibres and carbohydrates to refill the energy stores.
Should you eat after exercise?
Yes, but without the rush. After training, your muscles are more receptive to protein for at least 24 hours, and the glycogen stores refill at around 5% per hour. A skipped meal is no drama, but over time it costs you recovery and training quality. If you ate before exercise, you have until your next normal meal; if you trained fasted, you should eat something within an hour. So eating or not eating is the wrong question. The right one is: what and how much.