Specific or general
Do we mean one particular thing, any one, or the whole kind?
Meaning
We can talk about one particular thing, any member of a group, or a whole kind of thing. This shows whether choosing a particular member matters. Languages express these meanings with articles, pointing words, other words, or context; some have no articles. Unlike New and known information, this is not about whether the listener already knows what is being talked about.
How to spot it
Ask whether the speaker has a particular person or thing in mind, would accept any member of the group, or is talking about the kind as a whole.
In your native language
English uses articles such as a/an and the, along with words such as this and any, to show which thing is meant. A/an can present one member of a group without itself identifying which one, even when the speaker has a particular one in mind. To talk about a whole kind, English commonly uses a plural noun without an article, though other patterns are possible.
I'm looking for a book I left here.
The speaker has a particular book in mind, even though the noun uses a.
You can use any chair.
No particular chair is required.
Cats need a lot of sleep.
Cats without an article refers to the kind in general.
In your learning language
Spanish uses articles and pointing words to distinguish particular things, unspecified members of a group, and whole kinds. Un/una can refer to a particular item or any suitable one; context makes the difference. Definite articles such as el and los commonly mark whole kinds, while plural nouns without an article can refer to unspecified things.
Esta mochila es de Ana.
This backpack belongs to Ana.
Esta points to a particular backpack.
Necesito una mochila para el viaje.
I need a backpack for the trip.
This can mean any suitable backpack, not one already chosen.
Los gatos duermen muchas horas.
Cats sleep for many hours.
Los gatos refers to cats as a whole kind.
Learners whose language uses bare nouns for whole kinds may omit the definite article. Learners whose language allows a bare singular noun for an unspecified item may omit un/una where Spanish normally needs it.