adding custom fields?

hello, i want to add a hidden field to every new post i make. this is because:
when someone clicks the link of the post i want to use the “comments” page to load thumbnails. i already put away all the comments stuff but now i need a hidden field to tell the script which thumbnails to load (since every post will generate different thumbnails)
so i thought the built in “custom fields” would do the trick, but they are not submitted to the html output. they are in the sql but not in my html. so, what are the custom fields about when they dont generate any output?