Kód je na stěně.

Because stěna is the basic dictionary form, but after na when it means a static location (on somewhere), Czech uses the locative case.

So:

• stěna = wall
• na stěně = on the wall

Here, stěně is the locative singular form of stěna.

Here, na means on.

So na stěně means on the wall.

A useful contrast is:

• na stěně = on the wall
• ve stěně = in the wall

So if something is attached to the surface, Czech normally uses na.

Czech does not have articles like a, an, and the.

So Kód je na stěně can mean:

• The code is on the wall
• A code is on the wall

The exact interpretation depends on context.

Je is the 3rd person singular present form of být (to be).

So:

• je = is

In this sentence:

• Kód je na stěně = Code is / The code is on the wall

Unlike in some other Slavic languages, Czech normally does use the present-tense form of to be in ordinary sentences like this.

Yes. Czech word order is fairly flexible.

You could also say:

• Na stěně je kód.

This still means essentially the same thing, but the emphasis changes:

• Kód je na stěně focuses more naturally on the code
• Na stěně je kód focuses more on the location or introduces what is there

So the original sentence is completely normal, but it is not the only possible order.

Because Czech changes the case depending on whether you mean location or direction/movement.

• na stěně = on the wall → location, so locative
• na stěnu = onto the wall → movement toward a surface, so accusative

For example:

• Kód je na stěně. = The code is on the wall.
• Píšu kód na stěnu. = I am writing the code onto the wall.

This is a very common Czech pattern with prepositions.

A simple learner-friendly approximation is:

• kód ≈ koht (with a long o)
• je ≈ ye
• na ≈ nah
• stěně ≈ roughly styen-nye

A few important points:

• ó is a long o
• ě does not mean a long vowel by itself; it usually affects the consonant before it
• final consonants in Czech are often devoiced, so kód is pronounced more like kót at the end

You do not need perfect pronunciation right away, but it is good to notice that ě behaves differently from a normal e.

ě is a special Czech letter. It usually signals that the consonant before it is pronounced in a softer way.

In stěně, it affects both:

• tě
• ně

So stěně is not pronounced as if it were plain ste-ne.

For beginners, the most useful thing to remember is:

• ě often makes the previous consonant sound softer
• it is very common in Czech endings and word forms

Not necessarily. Kód is a broad word, just like code in English.

Depending on context, it can mean things like:

• a code
• a passcode
• a program/code in computing
• a symbolic or written code

So in Kód je na stěně, the exact kind of code depends on the situation.

Yes, it is perfectly natural.

It is a simple, standard sentence with:

• a subject: Kód
• a verb: je
• a prepositional phrase of location: na stěně

It sounds like normal everyday Czech.