Αυτό που λες είναι σημαντικό.

Αυτό που is a very common Greek structure meaning what or more literally that which.

So:

• αυτό = this / that
• που = that / which

Together, αυτό που λες means what you say or the thing that you are saying.

Greek often uses this pattern where English would simply use what.

Examples:

• Αυτό που θέλω = what I want
• Αυτό που είδες = what you saw

Here, που is a relative particle introducing a clause that describes αυτό.

In Αυτό που λες:

• αυτό = the thing / that
• που λες = that you say

So που connects αυτό with the verb λες.

It does not mean where here, even though που can have other uses in different contexts.

Because Greek often expresses what in this kind of sentence with a fuller structure: αυτό που.

English compresses the idea into one word:

• What you say is important.

Greek typically expands it:

• Αυτό που λες είναι σημαντικό.

This is normal and very natural Greek.

Λες is the 2nd person singular present form of the verb λέω, meaning to say.

So:

• λέω = I say
• λες = you say
• λέει = he/she/it says

In this sentence, λες means you say.

Because Greek usually does not need to state the subject pronoun when the verb ending already shows who the subject is.

Here, λες already tells us the subject is you (singular).

So:

• λες = you say

You could add εσύ for emphasis:

• Αυτό που λες εσύ είναι σημαντικό.

But without emphasis, Greek normally leaves it out.

Because it agrees with αυτό, which is neuter singular.

In Greek, adjectives must match the gender, number, and case of the noun or pronoun they describe.

Here:

• αυτό = neuter singular
• σημαντικό = neuter singular

So the agreement is correct:

• Αυτό ... είναι σημαντικό.

Compare:

• Αυτή η ιδέα είναι σημαντική. = This idea is important.
  (ιδέα is feminine, so σημαντική is feminine.)
• Αυτό το θέμα είναι σημαντικό. = This issue is important.
  (θέμα is neuter, so σημαντικό is neuter.)

It is an adjective.

It describes αυτό που λες = what you say.

So the structure is:

• subject: Αυτό που λes
• verb: είναι
• predicate adjective: σημαντικό

In English, important also functions as an adjective here.

Είναι is the 3rd person singular (and also plural) present form of είμαι, meaning to be.

Here it means is:

• Αυτό που λες είναι σημαντικό. = What you say is important.

Some forms of είμαι:

• είμαι = I am
• είσαι = you are
• είναι = he/she/it is, they are

Yes. Greek word order is more flexible than English, though some versions sound more neutral than others.

The most neutral version here is:

• Αυτό που λες είναι σημαντικό.

You may also hear:

• Είναι σημαντικό αυτό που λες.

This puts slightly more focus on important.

Both are natural, but the original sentence is a very standard way to say it.

Yes, but it means something a little different.

• Αυτό που λες είναι σημαντικό = What you are saying is important / The thing you’re saying is important
• Ό,τι λες είναι σημαντικό = Whatever you say is important

So ό,τι has the sense of whatever / anything that, not just the thing that.

That difference is important.

Because Greek often places the adjective after είναι in sentences like this, just as English does:

• is important
• είναι σημαντικό

This is the normal position for a predicate adjective after the verb to be.

A natural pronunciation is roughly:

af-TO pu LES EE-ne see-man-dee-KO

A few notes:

• Αυτό has stress on the second syllable: αυ-ΤΟ
• λες is one syllable
• είναι is often pronounced something like EE-ne
• σημαντικό has stress on the last syllable: ση-μαν-ti-KO

It is neutral and natural in everyday Greek.

Because it uses λες, it addresses one person informally (you singular).

If you wanted to address someone formally, you would use λέτε:

• Αυτό που λέτε είναι σημαντικό.

That can mean:

• What you say is important (formal singular)
• or What you say is important (plural)

It is in the nominative because Αυτό που λες is the subject of the sentence.

The whole subject is:

• Αυτό που λες = what you say

And that subject is linked to:

• είναι σημαντικό = is important

So αυτό is nominative neuter singular.

Sometimes in context, yes, because λέω can overlap with English say in a broad sense. But the literal meaning is still What you say is important.

If someone is speaking and you want to respond to the content of their words, this sentence is very natural.

Depending on context, English might translate it as:

• What you’re saying is important
• What you say is important
• occasionally What you mean is important

But the Greek itself uses λέω = say.