The first thing to know about race is that it's no more than a social construction developed by people for the purpose of catergorization. For example, you might think someone were ridiculous if they said, 'Hey, that guy has blue eyes, he must be really smart.' Race really isn't any different than this. We as people have taken the level of melanin in one's skin to be a determinant of who they are internally. (By the way, white people have melanin in their skin as well). This system of using skin color to assign societal value has been at play in America since its founding. From Manifest Destiny* all the way through our current covert prejudice society, systemic racism has woven its way into the fabric of America, whether we like it or not. In addition to this, there are misconceptions around the synonymous use of the terms race and ethnicity. While race is a social construct, ethnicity has roots in cultural beliefs and traditions. People use these terms together all of the time, but it's important to understand the difference between the two. For example, a person can have a race of White/Black/Asian, but their ethnicity is Italian/African-American/Korean. Remember, one is solely describing skin color, while the other refers to a person's culture and the norms and traditions inherent in that culture.
*If you're not familiar with Manifest Destiny, I strongly suggest you look it up online. It will give you valuable information in regards to how early Americans viewed colonization of other people (notably people of color), and how the system we now know as racism in America gained so much traction.