I was curious about that too, having a few friends with one non-Latino parent. Assuming the U.S. Census Bureau hasn't changed their methodology since 2010, Hispanic/Latino origin seems to be based on self-identification in their census questionnaires, so people with some Latino heritage and who feel like they have some sort of connection to Latino culture (I'd assume).
It's also noteworthy that the group that they outnumber, "Non-Hispanic White alone persons", is defined as people who don't identify as Latino/Hispanic and who specified White as their only race in the questionnaire, so I'm not really surprised that it's a group that's barely growing.
I had no idea the U.S. was considered the second largest Spanish-speaking country in the world though. That's pretty cool!