Pintu depan itu dipasang kunci baru untuk keamanan.

di- marks the passive voice in Indonesian.

So dipasang means was installed / was fitted, while memasang means to install / to fit in the active voice.

In this sentence, the focus is on pintu depan itu rather than on the person who did the action.

Compare:

Mereka memasang kunci baru pada pintu depan itu.
= They installed a new lock on that front door.

Pintu depan itu dipasang kunci baru.
= That front door had a new lock installed.

Indonesian uses the passive very often, especially when the doer is unknown, obvious, or unimportant.

itu is a demonstrative, and after a noun it usually means that or sometimes the depending on context.

So pintu depan itu can mean:

• that front door
• the front door in question

In Indonesian, demonstratives normally come after the noun:

• rumah itu = that house
• orang itu = that person
• pintu depan itu = that front door

Because pintu depan means front door, where depan is modifying pintu.

But depan pintu usually means in front of the door.

So:

• pintu depan = front door
• depan pintu = in front of the door

This is a very important difference. In Indonesian, many modifiers come after the noun.

Because kunci baru is the thing being installed.

So the structure is roughly:

Pintu depan itu + dipasang + kunci baru
= The front door + was fitted/installed with + a new lock

English would usually need something like with here, but Indonesian often does not.

So even though the literal word order may feel unusual to an English speaker, the sentence is natural in Indonesian structure.

Here, kunci is a noun, meaning lock.

So kunci baru means a new lock.

This is different from the verb idea to lock. For that, Indonesian usually uses forms like:

• mengunci = to lock
• dikunci = locked

So:

• Pintu itu dikunci. = The door is locked.
• Pintu itu dipasang kunci baru. = A new lock was installed on the door.

So this sentence is not about locking the door; it is about installing a new lock.

Dipasangi kunci baru is also very natural, and many learners will hear that version a lot.

The suffix -i often suggests that something is being given or fitted onto a place/location. So:

Pintu depan itu dipasangi kunci baru
very clearly means
The front door was fitted with a new lock.

The sentence you were given, dipasang kunci baru, is still understandable and acceptable. But dipasangi can sound a little more explicit about the door being the thing that received the lock.

So both are useful, but dipasangi is often especially clear in this kind of sentence.

Because Indonesian often leaves out the agent in passive sentences when it is not important.

So the sentence does not say who installed the lock. It just says that the lock was installed.

If you wanted to include the doer, you could say something like:

Pintu depan itu dipasang kunci baru oleh tukang.
= The front door was fitted with a new lock by a worker.

But in normal Indonesian, leaving out oleh is very common.

It means for security or for safety reasons.

Here:

• untuk = for
• keamanan = security / safety

keamanan is a noun formed from aman, which means safe or secure.

So the phrase expresses purpose:

A new lock was installed for security.

This is a very common Indonesian pattern: untuk + noun

For example:

• untuk kesehatan = for health
• untuk pendidikan = for education
• untuk keamanan = for security

Not very naturally in this context.

Because kunci baru comes right after dipasang, the most natural reading is:

a new lock was installed on the front door

not

the front door was installed

If you wanted to say the door itself was installed, you would normally make that clearer, for example:

Pintu depan itu dipasang kemarin.
= The front door was installed yesterday.

So in your sentence, kunci baru strongly guides the interpretation.

A natural active version would be:

Mereka memasang kunci baru pada pintu depan itu untuk keamanan.

This means:

They installed a new lock on that front door for security.

You could also hear:

Mereka memasang kunci baru di pintu depan itu untuk keamanan.

So the passive sentence is just shifting the focus away from they and onto the front door.

That kind of active/passive switching is very common in Indonesian.