Thức ăn thừa là rác hữu cơ.

It follows a very common Vietnamese pattern:

X là Y

• Thức ăn thừa = the subject
• là = linking verb, like is / are
• rác hữu cơ = the category or identity being assigned

So the sentence is literally structured as:

leftover food + is + organic waste

This pattern is used when one noun phrase is being identified as another noun phrase.

Yes, thức ăn means food.

A useful thing for learners: treat thức ăn as a fixed expression. Even though ăn means to eat, learners usually do best by memorizing thức ăn as one chunk meaning food.

A few notes:

• thức ăn is neutral and common
• đồ ăn is also very common, especially in everyday speech
• thức by itself is not usually used in normal conversation to simply mean food

So in this sentence, thức ăn thừa means leftover food.

Here thừa means left over, extra, or surplus.

In Vietnamese, modifiers often come after the noun they describe. So:

• thức ăn = food
• thức ăn thừa = leftover food

This is very normal Vietnamese word order.

You can compare:

• cơm thừa = leftover rice
• tiền thừa = extra money / leftover money

So if you are expecting English-style order like leftover food, Vietnamese usually flips that to food leftover.

Là is the linking word that connects two noun phrases. In this sentence, it works like is.

It is especially common when:

• identifying something
• defining something
• classifying something

So here, the sentence is classifying thức ăn thừa as rác hữu cơ.

That is why là sounds natural.

A helpful contrast:

• Thức ăn thừa là rác hữu cơ.
  Leftover food is organic waste.

But with adjectives, Vietnamese often does not need là:

• Rác này hữu cơ.
  This trash is organic.

So là is particularly important when the part after it is a noun phrase, not just a simple adjective.

Because Vietnamese usually puts the main noun first and the modifier after it.

So:

• rác = trash / waste
• hữu cơ = organic

Together:

• rác hữu cơ = organic waste

This noun-then-modifier order is very common in Vietnamese.

You can compare:

• nước nóng = hot water
• nhà lớn = big house
• rác vô cơ = inorganic waste

So rác hữu cơ is the natural Vietnamese order.

Here hữu cơ means organic in the environmental or biological sense: material that comes from living things and can decompose.

So in this sentence, rác hữu cơ means organic waste, such as food scraps, vegetable peels, and so on.

This is not necessarily the same as organic in the sense of certified organic farming. Vietnamese also uses hữu cơ in that context, but the meaning depends on the topic.

In this sentence, the intended meaning is clearly the waste-sorting meaning: organic waste.

Vietnamese nouns usually do not show singular/plural the way English does.

So thức ăn thừa can mean:

• leftover food in general
• leftover foods
• food waste as a category

The exact number is understood from context.

That means this sentence is best understood as a general statement:

Leftover food is organic waste.

Not necessarily one specific piece of food.

Yes, absolutely.

Both are natural:

• Thức ăn thừa là rác hữu cơ.
• Đồ ăn thừa là rác hữu cơ.

The difference is mainly tone and style:

• thức ăn = a bit more neutral or formal
• đồ ăn = very common and everyday

So if you hear either one, that is normal.

The tone pattern is:

• thức — sắc
• ăn — ngang
• thừa — huyền
• là — huyền
• rác — sắc
• hữu — ngã
• cơ — ngang

So one useful way to practice is chunk by chunk:

• thức ăn thừa
• là
• rác hữu cơ

A learner-friendly point: hữu may be tricky because of the ngã tone. In some southern accents, hỏi and ngã may sound merged or very similar, but the spelling still matters.

It is natural, especially in contexts like:

• recycling
• environmental education
• sorting trash
• school materials
• public signs or instructions

It sounds clear and normal as a general statement.

If someone wanted to sound slightly more formal or technical, they might say something like:

• Thức ăn thừa thuộc loại rác hữu cơ.
  Leftover food belongs to the category of organic waste.

But your original sentence is already perfectly good Vietnamese.