Post-processing is when you make adjustments to your images after you transferred them to the computer and before you upload them to the agencies. Photographers usually use Photoshop or Gimp for post-processing.
These adjustments could be color correction, white balance adjustments, removing blemishes on the skin of your model etc. Everything you change with your image after you took it is called post-processing.
Some photographers do very little post processing, others spend many hours on retouching their images before uploading them.
Ideally, you want to take your images with the correct white balance and proper focus so there is not so much you have to adjust later but that doesn’t always work and some images need more post-processing than others.
To get more information how the results of a post-processed image can look like, just do a search on Youtube with the keyword “photo retouching tutorial”. You will learn many tricks how to improve your own images.