This is a quite tricky theme, but it has got a simple answer.
"Gender" of words may have 2 main roles:



1) As far as living beings are concerned:



_In some cases, they are used to differentiate males from females in certain activities, job, professions and among the animal kingdom (similar to German in that case), hence avoiding ambiguities.

eg: "un conseiller" (masculine) vs "une conseillère" (feminine): both mean "councellor", however, the gender of the subject is known. But that is not always the case, and quite a good deal of words are still limited to their masculine form, which leads a few to speak of "language sexism"



2) As far as objects, actions, ideas and concepts are concerned:



_The second and perhaps most important use of masculine/feminine words, however, is totally subconscient and most French people themselves do ignore it completely.

"Feminine" words in French are more general, vague, they express categories, species, groups of ideas, whereas "masculine" words are specific, precise and accurate, they refer to the things themselves.


You can define a masculine word with a feminine one, because that word necessarily belongs to a wider category of ideas, but you can hardly ever do the opposite, because if the language were a river, that would be like going against the main stream. It goes from broader (feminine) to thinner (masculine) like in a funnel, and defining a larger theme (feminine) with only one of its elements (masculine) would be either wrong or incomplete.



Eg: "Une boisson" is the feminine word for "a drink", but a drink may be "de l' alcohol" (alcohol), "du lait" (milk), "du vin" (wine), "du sirop" (syrup) etc... all of which are masculine words. Of course there are a few exceptions, but the idea's there.



Those, basically, are the 2 main functions of "word gender" in French.