The banana we eat comes from the banana tree. It is a tall slender tree with broad green leaves. Before it became that tall tree with broad leaves, it started very small with either tiny or no leaves at all. Then little by little, it grows into a tree with very long broad green leaves that could, sometimes, be used as umbrellas when it is raining.
The proverb means that not everything can be achieved at a go. Certain things are achieved little by little. Some rich and powerful multination companies and organizations we see today grew little by little into what they are today.
You can begin your multinational organization or company now with very little funds at a very small place in a very small building. Someday, through hard work and providing good quality service, it would grow into something, great and prosperous, you could not imagine.