Η αναχώρηση για την Αθήνα είναι στις οκτώ.

Η is the feminine singular definite article, so it means the.

It agrees with αναχώρηση, which is a feminine singular noun. In Greek, articles change according to:

• gender
• number
• case

So unlike English, Greek does not use one single form for the in every situation.

Αναχώρηση is a noun meaning departure.

A few useful things to notice:

• it is feminine
• it is singular
• its basic dictionary form is η αναχώρηση
• it comes from the verb αναχωρώ = to depart / leave

So Greek is using a noun here, just like English can say the departure is at eight.

Normally, για often means for, but in travel contexts it can also mean to / bound for.

So:

• το τρένο για την Αθήνα = the train to Athens
• εισιτήριο για την Αθήνα = ticket to Athens
• αναχώρηση για την Αθήνα = departure for / to Athens

In this sentence, για την Αθήνα gives the destination.

Because για is followed by the accusative case in Modern Greek.

So the noun phrase changes like this:

• η Αθήνα = nominative
• την Αθήνα = accusative

The noun Αθήνα itself stays the same here, but the article changes:

• η → nominative feminine singular
• την → accusative feminine singular

This is very common in Greek: the article often shows the case clearly.

Greek very often uses the definite article with place names, especially in ordinary speech and after prepositions.

So Greek naturally says things like:

• στην Αθήνα = to/in Athens
• από την Αθήνα = from Athens
• για την Αθήνα = for/to Athens

To an English speaker, this may feel extra, but in Greek it is normal.

Είναι means is and is the third person singular of είμαι = to be.

So the structure is:

• Η αναχώρηση ... είναι στις οκτώ
• The departure ... is at eight

Greek normally keeps είναι in a full sentence like this.
In some very short notes, signs, or headlines it may be omitted, but for a standard sentence learners should include it.

Because Greek usually says clock times with σε + article + number.

στις is the contracted form of:

• σε + τις → στις

So στις οκτώ means at eight o’clock.

This is the normal way to give the time:

• στη μία = at one
• στις δύο = at two
• στις οκτώ = at eight

This is a very common learner question.

Greek treats clock times as if the word ώρες = hours is understood:

• στις οκτώ (ώρες) = at eight (hours)

That is why Greek uses the plural article for most hours:

• στη μία = at one o’clock
  
  • singular, because one hour
• στις δύο / τρεις / οκτώ = at two / three / eight o’clock
  
  • plural, because two hours, three hours, eight hours

So the plural form is grammatical, even though English simply says at eight.

The given order is the neutral, natural order:

• Η αναχώρηση για την Αθήνα είναι στις οκτώ.

But Greek word order is fairly flexible, especially for emphasis. For example:

• Στις οκτώ είναι η αναχώρηση για την Αθήνα.

This puts more focus on the time.

So the original sentence is not the only possible order, but it is a very normal one.

A simple pronunciation guide is:

ee ana-HO-ree-see ya tin a-THEE-na EE-ne stis ok-TO

More closely:

• Η = ee
• αναχώρηση = ana-HO-ri-si
• για = ya
• την Αθήνα = tin a-THEE-na
• είναι = EE-ne
• στις οκτώ = stis ok-TO

The stressed syllables are:

• αναχώρηση
• Αθήνα
• είναι
• οκτώ

Yes. Αναχώρηση is a general word for departure.

By itself, it does not tell you whether it is:

• a flight
• a train
• a bus
• a ferry
• some other departure

That usually comes from the context. If needed, Greek can be more specific:

• η πτήση για την Αθήνα = the flight to Athens
• το τρένο για την Αθήνα = the train to Athens
• η αναχώρηση του λεωφορείου = the departure of the bus

So in your sentence, αναχώρηση is a broad timetable-style word.