Post-workout means after your workout. Therefore, the term "post-workout meal" means your meal after the workout. This meal plays an essential role if you want to build or maintain muscle.
This is because you consume a lot of energy during a strenuous training session. Your glycogen stores are, therefore, no longer full at the end of the workout. To fill them, your body needs carbohydrates. Furthermore, a sufficient intake of protein after training is essential to protect your muscles from atrophy and to promote growth.
It's best to turn to fast-digesting sources of protein and carbohydrates. This is because the macronutrients in these foods will be quickly available to your body. The goal is to move your body from a catabolic state to an anabolic one.
For example, as a fast-digesting protein source, you could take Whey protein, and as a fast-digesting carbohydrate source, you could eat a banana or dextrose in a Whey shake. You do not need to consume these foods immediately after training, despite the common myth of a half-hour "post-workout window" after which your workout will have been useless if you do not drink a protein shake. However, it is recommended that you have your post-workout meal within 2-3 hours of your workout. The longer your workout is, the sooner you should provide your body with post-workout nutrients.
However, more important than the timing of the post-workout meal is that you eat a meal at all. Going a long time without eating after a workout is not advisable. This would negatively affect your muscle growth and, in the worst case, even lead to muscle loss.