Brašno je u kesi.

• brašno = flour; it is the subject of the sentence
• je = is; the 3rd person singular present form of biti (to be)
• u = in
• kesi = bag in the locative singular

So the structure is basically:

brašno + je + u kesi
the flour + is + in the bag

Because the preposition u means different things depending on whether it shows:

• location → it takes the locative
• movement into something → it takes the accusative

Here the sentence describes where the flour is, so Serbian uses the locative:

• kesa = nominative singular
• kesi = locative singular

So:

• Brašno je u kesi. = The flour is in the bag.
• Stavljam brašno u kesu. = I am putting the flour into the bag.

Notice the change:

• u kesi = in the bag
• u kesu = into the bag

Kesi is locative singular.

The noun is kesa (bag), a feminine noun ending in -a.
Its basic singular pattern is:

• nominative: kesa
• genitive: kese
• dative: kesi
• accusative: kesu
• vocative: keso
• locative: kesi
• instrumental: kesom

In this sentence, u expresses location, so kesi is in the locative.

Serbian normally has no articles like English a/an/the.

So brašno can mean:

• flour
• the flour

And u kesi can mean:

• in a bag
• in the bag

Which one is meant depends on context. In many situations, Brašno je u kesi would naturally be understood as The flour is in the bag or Flour is in the bag, depending on what people are talking about.

Brašno is neuter.

A very common clue is the ending -o, which often marks a neuter noun in Serbian.

That matters because adjectives, pronouns, and some past-tense forms must agree with it. For example:

• Belo brašno = white flour
• Brašno je sveže. = The flour is fresh.

Here belo and sveže are in neuter form to match brašno.

A simple English-friendly approximation is:

BRAH-shnoh yeh oo KEH-see

A few useful pronunciation notes:

• š sounds like sh in shoe
• j sounds like English y in yes
• e is usually like e in met, not like English ee
• Serbian words are usually pronounced quite consistently as written

So:

• Brašno ≈ BRAH-shno
• je ≈ yeh
• u ≈ oo
• kesi ≈ KEH-see

Yes. Serbian word order is fairly flexible, although the most neutral version here is:

Brašno je u kesi.

Other orders are possible, for example:

• U kesi je brašno. = The flour is in the bag.
  This puts more focus on where it is.

Because Serbian uses case endings, word order can move around more freely than in English. Still, not every order sounds equally natural in every context.

Je is the present-tense form of biti (to be) for he/she/it is.

Since brašno is singular, Serbian uses singular je:

• brašno je = the flour is

Compare:

• Brašno je u kesi. = The flour is in the bag.
• Knjiga je na stolu. = The book is on the table.
• Jabuke su u kesi. = The apples are in the bag.

In the last example, su is used because jabuke is plural.

Serbian distinguishes different kinds of location:

• u = in, inside
• na = on, at, on top of, and sometimes at certain places

Because the flour is inside the bag, Serbian uses u:

• Brašno je u kesi. = The flour is in the bag.

Compare:

• Knjiga je na stolu. = The book is on the table.
• Novac je u džepu. = The money is in the pocket.

Kesa usually means a bag, very often a plastic bag or shopping bag in everyday speech.

Depending on context, Serbian may use other words too, for example:

• torba = bag, handbag, school bag, larger bag
• vreća = sack
• paket = package
• džak = large sack

So u kesi often suggests in a bag, especially a lighter everyday bag.

Yes, it is completely natural.

It is a simple, basic location sentence of a very common Serbian type:

[thing] + je + u/na + [place]

Examples:

• Mleko je u frižideru. = The milk is in the fridge.
• Knjige su na stolu. = The books are on the table.
• Telefon je u torbi. = The phone is in the bag.

So Brašno je u kesi is a normal and useful sentence pattern to learn.