What are you drinking?
I am drinking the water....that was on the table, that we just talked about tasting strange, that cost money so will be put on our tab.
I am drinking water.....some water, that holds no interest for us other than it is water. It may be the water we just talked about or it may not because we don't care which it is in this part of the discussion.
In most situations English speakers don't really care and so don't make the point. But French speakers have an obligation to make it clear for the listener/reader so they have to include the appropriate article or modifier.
Every noun in French has an assigned gender. Fortunately it is binary. They are either masculine of feminine. All adjectives modifying that noun must match it in gender and number. Petit is an adjective. The masculine form of petit is actually petit. The feminine form of petit is petite.
All nouns in French are gendered in a way that has no logic as we would currently apply it. That means you have to remember what the gender of every noun is in French. There are some guidelines to help you with that. Unfortunately, there are so many exceptions that most of them don't work.
Nouns for things which actually do have a gender such as man, woman, dog, cat, horse carry the rule which accounts for their gender. In this Duo example both the noun and the adjective have a masculine form because the dog itself has an assigned gender. The spelling of the words tell you which one it is. Petit chien makes it masculine. Petite chienne would make it feminine.