I have hit the post limit of 1000 characters, so I am replying to my own post to add to it.
I want to now talk about natural selection. Now that you know that evolution is changing allele frequencies in a population over time, it will start to make a lot more sense once you understand the process by which allele frequencies change. There are actually a few different ways that allele frequency in a population can change, but the primary driver is selection. The different ways that allele frequencies can change:
-Mutation of one allele to another
-Migration into or out of the population
-Genetic drift (essentially random chance)
Selection is a broad term. It can be broken down into multiple different types of selection: natural selection, sexual selection, etc, but ultimately they all have the same effect. Selection is simply the process of changing allele frequencies in a population by causing individuals with a certain allele or alleles to have lower fitness. Fitness is another key term in evolutionary biology. It does not mean physical fitness like having a good body or being strong. It refers to an organisms chances of replicating their genome. A big component of fitness is survival, because to replicate your genome you need to survive until you can reproduce. Evolution does not cause organisms to survive better, it causes them to reproduce better. It is all just driven by which individuals replicate their genome and which do not, so the genomes that have a higher chance of being replicated are replicated. It is actually quite eloquently simple. There are adaptations that increase an organisms chances of reproducing, but actually decrease their chance of survival after doing so. Evolution doesn't "care", for lack of a better word, about survival.
Selection on a particular trait is just what logically happens when three criteria are met:
-variation of that trait across different individuals
-heritability of that variation to offspring
-differential fitness based on that variation