What does it mean when you see a full developed fruit on a plant
When you see developed fruit on a plant, you can safely assume that the plant is in the process of completing its reproductive cycle. Fruits develop from the ovary tissues of the plant, and are used for protection and dispersal of the plant's seeds, which contain the baby plants to begin the next generation.
Therefore, if you see a fruit, you know that at some time in the past, there was a flower on that location in the plant. You also know that the flower had an ovary, the female part. (It may have had both male and female parts, as many plants do.) You can also be sure that the female part of the flower was pollinated, which means that pollen, which contains plant sperm, reached the stigma, which is a receptor organ found on the female part of the plant. The sperm reached the eggs, which grew into a seeds. You can tell how many eggs were fertilized by counting the seeds inside the fruit.