Think "baby steps"! Anything we want to accomplish, be it a project, product, or something else, can be broken down into smaller steps. For example, when we want to go camping, we may need to make the site reservation, pack our tent and gear, travel to the site, pitch the tent when we get there, break camp, and travel back home. As we think about these tasks, each is comprised of its own steps. So when we pitch the tent, we may have to put the stakes in the ground, set up the frame, and tie down the canvas.
So initially, the WBS or PBS is nothing more than a hierarchical grouping of the tasks that help us meet our goal. Initially, it might be too difficult to estimate the costs and time associated with pitching the tent, but when we look at the smaller details, it should become more apparent and we can begin to form opinions of the time and costs that will be consumed.