Learn Hardcore Norwegian: Navet lager en lyd.

The noun nav is neuter, so its definite ending is -et.
English uses a separate word, the, but Norwegian often puts definiteness at the end of the noun instead.

Is navet singular or plural?

It is singular.

Here is the pattern:

et nav = a hub
navet = the hub
nav / naver or other plural forms depending on usage/context = plural forms

So in this sentence, we are talking about one hub, not several.

Why do we say en lyd and not et lyd?

Because lyd takes the article en in Bokmål.

en lyd = a sound
lyden = the sound

Norwegian nouns have grammatical gender, and the article depends on the noun:

en for many masculine/common-gender nouns
et for neuter nouns
ei for feminine nouns in forms that use it

So lyd goes with en, not et.

What exactly does mean here?