Spanish is a language that has words that are masculine and words that are feminine. It is very important to know which words are feminine and which words are masculine, because otherwise it's very confusing and sometimes we don't even understand what the people are telling us if we don't know the difference.
How does this work? Nouns, meaning names of things or ideas, can be either feminine or masculine. When we're talking about animals or people it's very easy - gender means sex. So this is how it works. Una nina means a girl, una nina. Notice it finished with an "ah" sound, una nina. Now, if I say un nino, it means a boy. Un nino. Ends with an "o."
Now, some words that are not people or animals have a set gender, and that gender is completely arbitrary. It just happens to be so because of the sound of the word. So for instance, the word for table, mesa, that's a feminine word because it ends in the sound "ah," mesa.
So this is the rule. Most words ending in the sound "o" are masculine, like libro, book, auto, car. Most words ending in the sound "ah" are feminine, like casa, house, or moneda, coin. Most words ending in the sound "yon" are feminine, such as situacion, like situation in English, situacion, and accion, action. These are feminine words. And all the words ending in the sound "odd," spelled ad, are feminine too. Sociedad, society, and ciudad, city. They're all feminine words.