Retrocedeix una casella.

Retrocedeix comes from the verb retrocedir, which means to go back, to move back, or to retreat.

In this sentence, retrocedeix means move back or go back.

Because the sentence is giving an instruction, it is best understood as an imperative:
Retrocedeix una casella. = Move back one square.

Here, retrocedeix is the affirmative imperative form for tu of retrocedir.

Catalan often uses this form to give a direct instruction to one person:

• Retrocedeix = Move back
• Escolta = Listen
• Obre = Open

So if you are talking to one person informally, retrocedeix is the correct form.

Yes. On its own, retrocedeix can also be the 3rd person singular present indicative:

• Ell retrocedeix = He moves back
• Ella retrocedeix = She moves back

So the form itself can mean either:

• you move back! (imperative, informal singular)
• he/she moves back (present tense)

The context tells you which meaning is intended. In a board game or instruction, it is naturally understood as a command.

Una casella means one square, one space, or one cell, depending on context.

In board-game language, casella usually means a space on the board.

So:

• una casella = one space
• dues caselles = two spaces

Because casella is a feminine noun in Catalan.

That means it takes:

• una = a / one for feminine singular nouns

Compare:

• una casella
• una carta
• una porta

But with a masculine noun, you would use un:

• un llibre
• un dau

It can suggest both, just like in many similar expressions.

In this sentence, una casella is best understood as one square / one space, because the number matters.

So although una can sometimes mean a, here the sense is clearly one.

Because the idea of back is already built into the verb retrocedir.

So:

• retrocedir = to move back / go backward

That is why Catalan does not need a separate extra word here.

English often uses a verb + adverb combination, such as go back or move back, but Catalan can express the same idea with a single verb.

Yes. This is a very normal Catalan word order for a command:

• Verb + object

So:

• Retrocedeix una casella.
• Agafa una carta.
• Tira el dau.

This is similar to English imperatives:

• Move back one square.
• Take a card.
• Roll the die.

A practical approximation is:

reh-troh-seh-DEYSH or reh-troh-seh-DESH

The most important points are:

• the stress is on the last syllable: -deix
• the ending -eix sounds like sh
• the r is Catalan/Spanish-style, not the English r

You do not need a perfect accent at first; just make sure the stress falls on deix.

Very often, yes. In games, casella is a common word for a square or space on the board.

It can also mean a cell or box in other contexts, such as a grid, table, or form.

So the exact English translation depends on context:

• board game: space / square
• grid or table: cell
• form: box

If you are speaking informally to more than one person, you would use the vosaltres imperative:

• Retrocediu una casella. = Move back one square. (to more than one person)

If you are speaking formally to one person, you would also use:

• Retrocedeixi una casella.

So the original Retrocedeix una casella is specifically the informal singular command.