Aromantic = does not experience romantic attraction.
Most of the time you see aromantic alongside asexual, for people who identify as both--aromantic asexuals. But there are aromantic people who aren't asexual, just like there are asexual people who experience romantic attraction, AKA Robbie and Orson!
What does this mean exactly to Martin? He is sexually attracted to women and acts on this sometimes--one night stands, friends with benefits, etc--but has no interest in romantic relationships because he doesn't feel that kind of attraction with anyone (there are other reasons why a person might not be interested in having romantic relationships so this part is key). It's that simple :)
EDIT: I should add that Martin is also very open to something like a queerplatonic relationship...and may...already be in one...*wink wonk*
Here are some resources if you're interested in learning more!
SECOND EDIT: If you're a Patreon patron, you can now read a 2000 word short story about Martin & Neve when they were kids! Pledges as low as $1 per month can read "Green Magic" today! If you're not already a patron, click the big orange Patreon logo on the sidebar to check out the rewards! (looks like a big swirly P!)