Parental investment is like when your mom and dad take care of you and help you grow up big and strong. Just like how they feed you healthy food, teach you how to walk, talk, and read, and make sure you are safe and healthy, animals do the same for their babies.
Some animals invest more in their babies than others. For example, some animals lay hundreds of eggs and leave them to fend for themselves, while other animals have only one baby at a time and take care of them for a long time. The amount of work and energy that parents put into making sure their babies survive is called parental investment.
Parental investment can be in many forms such as providing food, building nests, grooming, protection, teaching, and even sacrificing for their young. Taking care of their young can be very hard work, but parents do it because they want their babies to live and grow up to have babies of their own someday.