This question has plagued me for ages. Some people say he eats the heart container while most people are as clueless as me. Does anybody have any answers.
I imagine they are containers within which he stores hearts. I don't think he eats them.
Every time link loses a heart it is symbolic for he has to eat a heart to keep his health. The containers are simply the Tupperware in which he stores his enemies hearts for later consumption.
I say he absorbs there life force and the heart containers allow him to hold another one like @Deus was saying. This also explains how grass can drop hearts, they dont have hearts but they do have life force.
I kind of always figured that he absorbs them through some kind of magical ability or magical attribute of the heart container itself. I mean, he could eat them too I guess, but I don't think that the taste of heart would be all too appetising, you know? I'll stick to bacon personally.
Link actually consumes the raw fresh of his enemies with his Master Silverware. The only reason it isn't shown is because it would raise the ESRB rating of the games and hurt sales.
I tend to assume hearts and heart containers are really some of the very few plain old video game representational items for the players and not necessarily something that actually exists in universe.