In English there's a pretty big difference between "my kid brought the dog home from the park" and "my kid brought a dog home from the park."
In the first situation you're talking about a specific dog — probably your dog. In the second situation it's just a general, non-specific dog — possibly a stray. (And your child is likely to get a telling off.)
That's the difference between the definite and indefinite article. In English we use "the" when the thing we're talking about is familiar to us. We use "a" or occasionally "some" when we're talking about less specific things.
Spanish also makes the distinction between definite and indefinite articles, but with an extra twist: They change according to the gender of the noun you're talking about, and how many of them there are.
Let's see how it works.
First let's meet the definite articles. Remember these are the equivalents of "the", and you use them when you're talking about a specific thing, rather than a general kind of thing. (E.g., you're driving the car, vs driving a car.)
Here the word "contract" is masculine, and we're only talking about one of them. So we use the masculine, singular article: el.
Warning: Be very careful not to confuse the definite article "el" (the) with the pronoun "él" (he). So close, but so different! They are pronounced exactly the same, they just differ in the written form.
We're still talking about that masculine noun, "contract", but now there is more than one contract. Busy day at the office! So we use the masculine plural article: los. Our contrato also turns into a plural, so it gains an -s. (If you're not sure how to make plurals, see our lesson on making plurals in Spanish.)
"Bottle" is a feminine noun in Spanish, and here we're only talking about one bottle of wine. So we use the feminine singular article: la.
More than one bottle of wine broke in this upsetting example, so we use the feminine plural article: las. Our botella also gains an -s because it's now plural.
Now let's meet the indefinite articles. Remember these are the equivalents of "a" or "some", and you use them when you're talking about a general kind of thing. (E.g., you're eating a banana, rather than eating the banana.)
We're talking about those contracts again. Remember a "contract" is a masculine noun in Spanish. Again, there's just one of them, so we're going to use the masculine, singular indefinite article: un.
Now there's more than one contract. It's still masculine (even though we've thrown in the confusingly female María), so we use the masculine plural indefinite article: unos.
Back to the wine! Here we've got Sofía drinking a whole bottle of wine, which is a feminine noun. Luckily she only drank one bottle, so we're going to use the feminine singular article: una.
And here we've got some good friends who brought multiple bottles of wine to add to our wine collection. There's more than one bottle, and it's still a feminine noun, so we use the feminine plural article: unas.