The Zone of Proximal Development is the sweet spot between what a child can do on their own and what they cannot do, even with help. It is in this area that a child can learn a new skill with the assistance of a someone else.
As a parent, it is important to find the ZPD to foster the learning process and to avoid frustration. To do this you must be aware of readiness signals. For a baby that's getting closer to being ready for solids, this could be being able to hold their head up with good control. For a child approaching learning to use a pencil, they should be able to grasp and control objects with their hands. Trying to introduce new skills before a child is ready can make learning more difficult than it needs to be. If you try to teach them too early, they won't learn, even with your help. Watching for these signals can help you from interfering with tasks a child can do on their own, or having unrealistic expectations of their abilities.
Once you have identified a skill within the Zone of Proximal Development, it is your job as a parent or caregiver to scaffold. Scaffolding is is offering support, encouragement, and guidance to the child as they learn the task. Allow the child to do as much of the task that they capable of on their own and offer support on the parts that are too difficult for them. Again, watch for signals and use your intuition to decide what they are ready for. An example would be teaching a child how to ride a bike. They may be able to steer and peddle on their own, but need help with balancing. You can provide support, by holding the seat to steady them until they are ready to balance by themselves.
Through scaffolding your child, and paying attention to their readiness signals, you can help them learn and reach their full potential.