What is evolution and natural selection?

That's a pretty long discussion, but I'm going to simplify it pretty hard.

Evolution is the changes that a thing (plant, animal, etc) goes through over an extremely long time, mainly due to mutations. Blue eyes in humans did not originally exist, and were a mutation. That person with the mutation had children, and spread the blue-eyed gene.

Natural selection is the idea that things (plants, animals, etc) evolve to fit a certain role, and that things that do not fit this role either adapt or die.

For example - A bug may be running out of food in their environment. The bug has a few choices - go extinct, evolve to eat a different kind of food in their current environment, or travel to a new environment that has their food. The first two options would be natural selection - the existing bug no longer fits in the world anymore, and must adapt.