Flowers have male and female parts to enable them to reproduce (make new plants).
Once a flower grows, it needs to be pollinated. The most common pollinators are insects (for example bees, wasps and butterflies). Insects do not intentionally pollinate flowers, they move from flower to flower and pollen catches on their bodies as they go. This pollen is then transferred from one plant to another, pollinating the flowers. Over time, flowers have evolved so that they attract particular insects which are more successful at pollination. Again, this has not happened deliberately. For example, a bee may be attracted by a particular scent (smell). Flowers with that smell are more likely to be successful at pollination than other flowers and therefore more likely to reproduce. The seeds that they produce are also likely to have a stronger smell and so flowers change over time.
This video shows tomato plants growing. Focus on how the flower becomes a fruit. It is a Youtube video, so please watch it with adult supervision.
Trees and grasses often pollinate by the wind. Catkins are an example of flowers which are blown from one plant to another for pollination.
Bats and birds also sometimes pollinate flowers. This is a less successful way to pollinate and therefore flowers that use this method tend to flower throughout the year to increase their chances of success. It is too cold in this country for plants to flower in the winter months, so this is not a common method in this country. Hummingbirds are an example of birds which pollinate flowers in this way. Again, it's a Youtube video, so please watch with supervision.