На катушке мало ниток.

In Russian, the present tense of to be is usually omitted.

So На катушке мало ниток literally looks like:

• На катушке = on the spool
• мало = little / few / not much
• ниток = thread(s), in the genitive plural

Russian does not normally say a separate word for is here.
If you wanted the past or future, you would use a form of to be:

• На катушке было мало ниток. = There was little thread on the spool.
• На катушке будет мало ниток. = There will be little thread on the spool.

Because after на meaning location, Russian uses the prepositional case.

The base form is катушка.
In the prepositional singular, it becomes катушке:

• катушка → на катушке

So на катушке means on the spool or on the bobbin/spool.

Compare:

• на столе = on the table
• на полке = on the shelf
• на катушке = on the spool

Because the thread is thought of as being on the spool, wound around it.

This matches English fairly well, since English also says on the spool. Russian uses на with many surface/location meanings, and a spool is treated as something a thread sits or is wound on.

Also remember the motion/location difference:

• на катушке = on the spool, already there
• на катушку = onto the spool, movement toward it

Because мало normally requires the genitive case.

With countable nouns, мало usually takes the genitive plural:

• мало книг = few books
• мало людей = few people
• мало ниток = little thread / few threads

So the form ниток is there because мало governs it.

This is a very common Russian-English mismatch.

In English, thread is often a mass noun, so English says little thread.
In Russian, when talking about sewing thread, people very often use нитки in the plural for the material in general.

So:

• мало ниток sounds natural in Russian
• even though English may translate it as little thread

This does not necessarily mean separate individual threads are being counted one by one. It is just how Russian commonly expresses the idea.

Yes, ниток is the genitive plural of нитка.

• нитка → ниток

The form нитей belongs to нить:

• нить → нитей

These words are related, but not identical in usage:

• нитка is the everyday word for a thread, especially sewing thread
• нить can sound more literary, technical, or abstract depending on context

So in a practical sentence about thread on a spool, ниток is the normal everyday choice.

Мало means little, few, or not much/not many, but it often has the nuance of insufficient or less than desired.

So this sentence does not just state a quantity neutrally. It often suggests that there is not enough thread left on the spool.

Compare:

• мало ниток = little thread, not much left, probably less than needed
• немного ниток = a little thread, some thread, a small amount without the same negative feeling

Yes. Russian word order is flexible, and changing it changes the emphasis.

The most neutral version here is:

• На катушке мало ниток.

Other possible orders:

• Мало ниток на катушке.
  Emphasizes the small amount first.

• Ниток на катушке мало.
  Often sounds contrastive, as if you are specifically commenting on the amount of thread.

So the sentence can be rearranged, but the original order is very natural and neutral.

The stress is:

На катУшке мАло нитОк.

A rough pronunciation guide:

na ka-TOOSH-ke MA-lo ni-TOK

Stress matters in Russian, so it is worth learning the stressed syllables early:

• катУшка
• мАло
• нитОк