A custom field, also known as meta-data, or post-meta, is information such as strings, integers, randomly generated values, etc., that is related to a WordPress post.
These values are stored in the database, referenced later by their
meta_key and association to the originating post.
Here is an example of calling up a custom field value whose key is
echo 'Today I am eating <em>' . get_post_meta($post->ID, 'my_lunch_today', true) . '</em>';
Which may produce something like:
Today I am eating tuna salad
For more information on using custom fields, see: Custom Fields « WordPress Codex