Got a C- in gen chem my first semester of college. I had to retake it because pretty much every medical school requires a C or higher to meet the requirement. I didn't realize this was the case until talking to my advisor at the end of my junior year. Retook it that summer while studying for the MCAT and got an A+
In both of my interviews I mentioned how poorly I did and what I learned from my mistake. I had an upward trend the rest of my college career so it didn't sink me.
tl;dr a C- won't kill you, but you need to retake it if it's a required course for a med school