When you realize something, you become fully aware of what is happening, like when you realize all the people in your basement who are smiling at you and starting to sing to you are there because it's your birthday. Surprise!
Realize usually describes the moment your thoughts click, like the moment you close the door behind you, you realize you don't have your keys. You are locked out! So you knock on your neighbor's door. You realize, here meaning that you are fully aware, that she might not appreciate having to help you because she's trying to leave for work. You tell her your new goal is to be more organized. She says, until you realize, or reach, that goal, you better hide an extra key somewhere in your yard.