Using the Definite Article
The definite article in Spanish is sometimes used with nouns when the noun being referred to is separated from any specific, physical form of the noun and what is being referenced is the idea of the noun. This can be really tricky, specifically because definite articles are primarily used in English to talk about specific objects.
El perro tiene que salir afuera. - The dog has to go outside. (A specific dog is being discussed.)
Quiero comprar un perro. - I want to buy a dog. (No specific dog is in mind yet.)
A mí no me gustan mucho los perros. - I don't really like dogs. (Dogs as an idea.)
Here's a great article that might shed some more light on this topic for you:
It takes awhile to get used to using the definite article in this way, but don't worry -- practice makes perfect!