Learn Hardcore Spanish (Spain): No han llegado aún; habrá mucho tráfico en la autopista. - They have not arrived yet; there is probably a lot of traffic on the motorway.

Han llegado is the present perfect: they have arrived. In Spain, this tense is very commonly used for past events that are still connected to the present moment.

So No han llegado aún means They haven't arrived yet, with a clear link to now.

If you said No llegaron, that would usually sound more like They didn't arrive, referring to a finished past event, not an ongoing situation.

What does aún mean here, and is it the same as todavía?

Here aún means yet.

So:

No han llegado aún = They haven't arrived yet
No han llegado todavía = same meaning

In this sentence, aún and todavía are interchangeable.

A useful note:

aún with an accent often means still/yet
aun without an accent often means even

In everyday writing, though, you may sometimes see variation.

Why is there no subject like ellos?