"L'homme mange la viande."
"she eats meat" (without context) should be interpreted as "she eats (a piece of) meat". In French, that is the partitive case (part of = some), so to be translated with preposition DE + definite article :
she eats meat = elle mange DE LA viande
L'homme mange la viande/ the man eats the meat means that he eats the meat that is on his plate, or the one that was cooked for him or any other way of defining a specific meat as opposed to meat in general
the mean eats THE meat = l'homme mange LA viande