Do you know what a blueprint is? When we build something, we need to have instructions telling us what we need and how to put the parts together. A blueprint is a list of items that we need to build the thing and also a set of instructions for us to follow.
Let's talk about how to put the parts together and make a bicycle.
First of all, we need to know what a bicycle looks like. Do we know what handles look like? How many handles do we need to make a bicycle? How about chains (*1)? Do you know what chains are made of -- iron? plastic? And don't forget about the brakes. After getting a general idea of what we are making, we now know what parts we need to get to make a bicycle.