Learn Hardcore Hungarian: A jelerősség rossz.

In Hungarian, the verb to be is often left out in the present tense, third person when you are linking a subject to a noun, adjective, or adverb.

So:

A jelerősség rossz. = The signal strength is bad.

There is no spoken or written van here.

Compare:

Én fáradt vagyok. = I am tired.
Te fáradt vagy. = You are tired.
Ő fáradt. = He/she is tired.

So in the third person singular, Hungarian often just uses:

subject + adjective

That is exactly what is happening here.

What does a mean here?