What features are extracted from pre-trained model of CNN Keras?
Here is a good post about how features are extracted using CNN: https://towardsdatascience.com/cnn-application-on-structured-data-automated-feature-extraction-8f2cd28d9a7e

"I don't know what features are extracted from that." 
It depends on what dataset was that CNN pre-trained on. E.g. if the model was pre-trained on a dataset consisting of dogs and cats, the features should contain useful information about dogs and cats. 

In practice it turned out that models pre-trained on ImageNet dataset extract features which are a good starting point for training on other custom datasets.