There are two types of snakes. Their predators are big birds, like hawks and eagles. They eat small animals like mice and rabbits. They can crossbreed, and produce offspring. Unfortunately, not all offspring survive. About 3/5 of the offspring born, survive. There isn't always enough food for everyone, so not all offspring survive.
For each generation, not all the snakes are the same. Almost like how if your mom has blue eyes but your dad has brown eyes, and you get brown eyes but your brother gets blue eyes. Different traits are inherited from your parents. It all depends on which trait is dominant, and which is recessive. The dominant trait in this species is small, green snakes with big tounges. The differences between the same species are called variation. People aren't all the same. Maybe you have a different color hair. That is an example of variation
In this environment, there are two different variations of the same species of snake. They can cross-breed or breed with the same type of variation. There are big, brown snakes with short tounges, and small, green snakes with longer tounges. Sometimes there's not always enough food to go around, so the snakes compete for food. Natural selection selects against the brown snakes because their tongues are shorter so they have a harder time catching food. Predators also play a role in this. There is more grass than dirt, so they are easy to see when they go onto the grass.
Since there is more grass than dirt, the brown snakes have trouble moving around. They have to be careful and hide in the shadows of the trees. Food is also a problem. The hawks and snakes also compete for mice, though the hawks also eat the snakes. Since natural selection selected for firmer beaks, most of the population of hawks have hard beaks to penetrate the snakeskin, making it harder for the snakes to defend themselves. The green snakes are smaller and can pry themselves out of the hawk's talons easier, and this is another factor of natural selection.
Natural selection is the concept that those who survive, live to reproduce and pass on that trait, meanwhile, those who are less favorable die. For example, nature selected for the small, green snakes since they are smaller and harder to see, while the brown snakes are easier to see on the grass, and can't escape easily. Also, the green snakes have longer tounges, making it easier for them to catch food, and harder for the brown snakes. This means more of the population is green snakes
Since the brown snakes have the less favorable traits, they eventually all disappear. The entire population of the snakes then becomes the small, green, long-tongued snakes.