We use “realize” when we suddenly discover the reality of a situation that we were unaware of before, because something event happens and helps us understand it. "I realized that I should have brought an umbrella with me when it started to rain." "Realize" can also be used to show that you have achieved something. "By the age of 25, I realized my ambition to become a doctor." We use “understand” when we can comprehend information or a situation. "I didn't understand today's lesson until the teacher explained it for the second time."

Hello understanding has a higher level of responsibility as per realizing and it has more emphasis on the subject than realizing.