In English, we have three articles - "a", "an", and "the". "The" is a definite article. "A" and "an" are indefinite articles. In other words, if you say "the table", you are referring to a specific table, but if you say "a table", which particular table doesn't matter.
We are lucky enough in English to only have to deal with these three. Spanish, which is supposedly the easiest foreign language for native English speakers to learn, has many more articles. In Spanish, one must not only distinguish between definite and indefinite, but also between gender and number. Esperanto, in contrast, only has one article, "la" which is the definite article. Esperanto does not use an indefinite article.
Suppose I want to say, "The boys and the girl played a game with the dog." Let's just look at the nouns
- The boys
- the girl
- a game
- the dog
- Los niños
- la niña
- un juego
- el perro or la perra, depending whether the dog was male or female.
- La knaboj
- la knabino
- ludon
- la hundo (male) or la hundino (female)
Esperanto, with only one article to remember, has an advantage over both English, Spanish, and many other languages.