Body shaming is someone trying to make you feel bad for having a body that doesn't conform to cultural (or just their own personal) beauty ideals.
Fat shaming is the most common example of that, but shaming women for not having big enough boobs (or have boobs that someone thinks are too big), not having a big enough butt, and various other kinds of body shaming for women are also common.
Normally men face less body shaming, but there are certainly exceptions, and for men it can sometimes be based on height, penis size, lack of muscles, or being fat.
As a trans woman, I can tell you that trans people also face a special kind of body shaming based on our ability to "pass" as cis people of our gender. People will sometimes feel the need to critique whatever they perceive as our failures in that regard.
Whoever you are, you are beautiful (or handsome, if you prefer) and you deserve to love your body the way it is. Some people need to change their bodies in order to love them better (like trans people with hormones and/or surgery, or other types of things), and if you need to do something that's ok too.
But whatever your body looks like, you are beautiful and deserving of love. I say this as a middle aged (45), 425 pound trans woman who doesn't think she passes particularly well -- whatever faults people might find with my body, I love it, and I love myself.
As for dealing with body shaming, I deal with it the same way I deal with any other person who is insulting.
On a good day I might try to educate them, on a so-so day I might ignore them, and on a bad day I might just snap at them sarcastically - but I strongly believe in not body shaming them right back, because I think it's wrong.
In any case, I know I'm beautiful to me, and I don't let body shaming insults hurt me the way they used to, because my own opinion matters and theirs doesn't. I know many people who love me just the way I am, and I know I can find love just the way I am.
So the opinion of this person doesn't matter to me, and I refuse to let them get in my head with their toxic opinions.