TV-MA is a more severe rating than R. TV-MA is the equivalent of NC-17 in the US. Note that R and MA are used by two different groups. The MPAA (Motion Picture Association of America) uses a rating system that you can review here where R is the second most severe, coming up behind NC-17.

M films are recommended for people aged 15 or older. A person needs to be 15 or older to see an MA 15+ film. The content is strong and unsuitable for people aged under 15. However people under 15 can see an MA 15+ film if accompanied by their parent or an adult guardian.