Кефир — один из самых простых продуктов для лёгкого завтрака.

In Russian, the present-tense verb to be is usually omitted. So where English says Kefir is one of..., Russian simply says Кефир — один из....

The dash helps show the link between the subject and the predicate. In this kind of sentence, it often stands where English would use is.

Because кефир is a masculine singular noun.

Russian один changes for gender:

• один = masculine
• одна = feminine
• одно = neuter

So since кефир is masculine, Russian uses один из... = one of...

A noun ending in a consonant is very often masculine in Russian. Кефир ends in -р, so it is masculine.

That is why you get masculine forms like:

• один
• простых продуктов describing a masculine noun phrase headed by продукт

Because Russian expresses one of as один из + genitive plural.

So:

• один из студентов = one of the students
• один из продуктов = one of the products/food items

In your sentence:

• один из самых простых продуктов = one of the simplest products / food items

Because из in this structure requires the genitive plural.

The pattern is:

один из + genitive plural

So:

• nominative singular: продукт
• nominative plural: продукты
• genitive plural: продуктов

That is why the sentence has один из самых простых продуктов.

Because самых простых has to match продуктов, which is genitive plural.

The basic superlative pattern is:

• самый простой продукт = the simplest product

But after один из, the whole phrase must go into the genitive plural:

• один из самых простых продуктов = one of the simplest products

So самый простой changes to самых простых.

Russian often forms the regular superlative with самый + adjective.

Examples:

• простой = simple
• самый простой = the simplest

In this sentence, the form changes because of case and number:

• dictionary/basic form: самый простой
• actual form here: самых простых

So the superlative is still built with самый, just inflected.

Because the preposition для requires the genitive case.

So:

• nominative: лёгкий завтрак = a light breakfast
• genitive after для: для лёгкого завтрака = for a light breakfast

Both the adjective and noun change:

• лёгкий → лёгкого
• завтрак → завтрака

It means a light breakfast—something not heavy, not very rich, not a large meal.

So для лёгкого завтрака means for a light breakfast or as part of a light breakfast.

Not in the business/marketing sense that English learners often think of first.

In Russian, продукт often means a food item, grocery item, or simply something you eat or drink. So here продуктов means something like:

• foods
• food items
• things to have

That is very natural Russian usage.

Here простой means something like:

• simple
• basic
• uncomplicated
• easy to use / easy to prepare

With food, it often suggests that it is convenient and does not require much effort. So один из самых простых продуктов means kefir is one of the easiest, most straightforward things to have for that kind of breakfast.

Yes, but the meaning changes.

• один из самых простых = one of the simplest
• самый простой = the simplest

The original sentence is less absolute and sounds more natural in many contexts. It says kefir belongs in that group, not that it is definitely number one.

Yes. Russian word order is fairly flexible.

The original:

• Кефир — один из самых простых продуктов для лёгкого завтрака.

is a neutral, natural order.

You could also say:

• Для лёгкого завтрака кефир — один из самых простых продуктов.

That version emphasizes for a light breakfast more strongly. The meaning stays basically the same, but the focus shifts.

Yes, ё is a separate letter from е, although in many texts Russians often write е instead of ё.

So you may also see:

• легкого завтрака

but it is understood as лёгкого завтрака.

Also, pronunciation does not match the spelling perfectly here:

• adjective endings -ого / -его are usually pronounced roughly like -ово / -ево
• and in лёгкого, the consonants also sound a bit different from what an English speaker might expect

So this is a good word to learn by ear as well as by spelling.