Jestem za.

Jestem is the 1st person singular present tense form of być (to be).

So:

• jestem = I am
• jesteś = you are
• jest = he/she/it is

In Jestem za, the verb tells you the subject is I, so the sentence already means I am ...

Polish often drops subject pronouns when they are obvious from the verb ending.

Because jestem already means I am, adding ja is usually unnecessary.

• Jestem za. = I’m in favor.
• Ja jestem za. = I’m in favor with extra emphasis on I

So the version without ja is the normal, natural one.

Here za means for in the sense of in favor of or supporting something.

So być za means:

• to be for
• to be in favor
• to support

This is a very common expression in discussions, votes, meetings, and everyday conversation.

Yes — it is the same word za, but here it has a different meaning.

Polish prepositions often have multiple meanings depending on context.
So za can mean things like:

• behind
• for
• in exchange for
• and, in expressions like być za, in favor of

That is normal in Polish and not something unusual.

Because the rest is often understood from context.

For example, if people are discussing a proposal, someone may ask:

• Kto jest za? = Who is in favor?

Then you can simply answer:

• Jestem za.

The idea or proposal does not need to be repeated if everyone already knows what is being discussed.

In this meaning, za is followed by the instrumental case.

So:

• Jestem za planem. = I’m in favor of the plan.
• Jestem za tym pomysłem. = I’m in favor of this idea.
• Jestem za zmianami. = I’m in favor of the changes.

This is important because za does not always take the same case in every meaning.
With być za in the sense of supporting something, think of it as:

• być za + instrumental

The usual opposite is:

• Jestem przeciw. = I’m against.
• Jestem przeciwko temu. = I’m against that.

A useful nuance:

• Nie jestem za = I’m not in favor
• Jestem przeciw / przeciwko = I’m against

Those are not always exactly the same.
Nie jestem za can sound weaker or less direct, while jestem przeciw is a clear statement of opposition.

It is neutral and very common.

You can use it:

• in everyday conversation
• in meetings
• in classroom discussions
• in votes
• in more formal situations

It is short, natural, and standard Polish.

A simple English-friendly approximation is:

• YES-tem zah

More roughly:

• jestem ≈ YES-tem
• za ≈ zah

A few details:

• j in Polish sounds like English y in yes
• e is like e in met
• a is like a in father but shorter

So Jestem za starts with a y sound, not a j as in English jam.

The normal order is:

• Jestem za.

You can add words for emphasis:

• Ja jestem za. = I’m in favor.
• Też jestem za. = I’m in favor too.
• Zdecydowanie jestem za. = I’m definitely in favor.

But Jestem za is the basic, natural form.
You generally would not move za to the front here.

Yes, very often.

Depending on context, Jestem za can sound like:

• I’m for it
• I’m in
• I support that
• Sounds good to me
• I’m on board

It is especially common when reacting to a suggestion or proposal. So even though the literal structure is short, the practical meaning can be quite broad.