Astral Projection is an out of body experience (OBE) where your spirit is known to leave your physical body to wander around in the astral dimension. Your spirit has a kind of connection with your body called "the silver cord".
Having a successful AP can take years of practice and requires strong patience. The site is full of methods of how to do AP successfully. Take your time and seek more info.
Astral projection is a process through which our consciousness leaves our physical body and is free to travel anywhere. That would be considered as the simplest definition.
And yes, it may take a long time before you can have an OBE.