In Indo-European languages, nouns are classified into three genders: masculine, feminine and neuter. Words that refer to nouns must agree with them for gender (e.g. in English, "he", "she" and "it").
Dutch has almost entirely merged the masculine and feminine genders into one. So, any given noun may be of neuter gender or of *common gender. Please ignore anyone who uses incorrect terminology like "male", "female", "neutral" or "not gendered". Those words have completely different meanings.
Use de with words of common gender, and het with neuter words. For example: de man, het dier.
In the plural, use de for both. For example, de mannen, de dieren.