Living things have the ability to produce more offspring than can possibly survive. This was noted by Charles Darwin in his theory of Natural Selection.
Organisms are subject to outside pressures that affect their ability to survive, including competition for space, mates, and resources, as well as the risk of becoming prey.
If a certain plant has a potential to produce a large amount of offspring, there is a greater chance or probability that more of them will survive to adulthood and reproduce themselves. When an organism successfully survives and reproduces, it is deemed to be "fit". Thus, if a plant can release many seeds, which are actually tiny embryos with a food supply, there will be a greater chance some will survive and thrive, creating the next generation.
Seeds are contained within fruits. Fruits are ripened ovaries that provide a certain amount of protection for the seeds they contain. When seeds are released from fruits, they have the ability to sprout into new plants. Animals which eat fruits can help spread seeds in their feces. These can then grow into the next generation. There are other adaptations for seed dispersal, including seeds dispersed by wind.
It is important to note that, since fruits and seeds can both be consumed as food for many animals, a plant producing a greater amount of fruits and seeds will lead to a higher probability some will survive to form mature offspring.