In Spanish, things like objects, animals, and even concepts can be grouped into different categories called genders. These genders are called masculine and feminine.
When we refer to an object or idea that is feminine, we use certain words to describe it, like "la" (meaning "the" in English) and "una" (meaning "a" in English). For example, if we were talking about a pencil in Spanish, we would say "el lápiz" (meaning "the pencil" in English) because "lápiz" (meaning "pencil" in English) is masculine.
On the other hand, if we were talking about a book, which is feminine, we would say "la libro" (meaning "the book" in English) because "libro" is a feminine word.
It's important to remember that not all words follow the same pattern and there are some words that don't fit into the masculine and feminine categories at all!
So, in summary, grammatical gender is the way that Spanish language groups things into two categories called masculine and feminine and it impacts the words used to describe those things.