het is used for 'common' nouns, and 'de' for neutral. in the past, Dutch had 3 grammatical genders, like german. But the masculine and feminine forms collapsed into one single category. It's important not to confuse grammatical gender with real, human genders or any notion of 'masculine/feminine' characteristics. you just have to learn what article matches each noun. it's important because the grammatical gender of a noun influences other parts of a sentence that must agree with it.
also, all plural forms use 'de' EVEN IF the singular form takes 'het'
it just is a way to refer to that noun category that USED to be 'masculine' and 'feminine' separated, but those are now joined, so it had to be re-named. it's just the translation. like how french has 'le masculin' et 'la feminine', it's just called 'common' and 'neuter' (to reflect how 'common' used to be 2 separate categories)
Please read Emmet's comment above.
The both mean the.
Het is used with neuter gender words.
De is used with common gender words and with plurals.
For more information and some guidelines to distinguish both groups of words, please read: