Hi, Mansur.
Welcome to DLS Course 4. The second assignment in Week 1 takes the knowledge that we learned about how Convolution and Pooling layers work in the first “Step by Step” assignment and then applies that to build a couple of actual models. But now we are moving to using the TensorFlow/Keras framework to build the models.
If you have questions, it will be better if you can ask something specific rather than just saying “Please explain assignment 2”. With that general a question, where do we start? 
If you would like a bit more background on how the Keras Sequential and Functional APIs work, here’s a good explanatory thread to start with. Please have a look at that and see if it sheds any light on how to proceed with this assignment. The other general suggestion is that the notebooks try to give pretty complete explanations, so it’s also a good idea just to read everything again from the start and see if it makes more sense the second time through. When you get to some specific point that is not clear, then you have a good question to pose here.