Guardamos el recurso de reposición por si lo desestiman.

Here guardar most naturally means to keep, to hold onto, or to save for later use.

So Guardamos el recurso de reposición suggests something like:

• We’re keeping the appeal
• We’re holding the appeal in reserve
• We’re saving it for later

It does not mean guard in the English sense, and it is probably not the computer meaning of save unless the context is explicitly digital.

It can be either, because for -ar verbs, the nosotros form is the same in:

• present: guardamos = we keep / we are keeping
• preterite: guardamos = we kept

You tell the difference from context.

In this sentence, present is the more natural reading, because por si lo desestiman points to a possible future event:

• We’re keeping the appeal in case they reject it

If it were clearly past, you would usually expect more past context, such as:

• Guardamos el recurso de reposición por si lo desestimaban

This is a fixed legal term in Spain.

A recurso de reposición is a type of administrative appeal in which you ask the same authority that made the decision to reconsider it.

A couple of useful points:

• recurso here does not mean resource
• the whole phrase should be learned as one legal expression
• in legal Spanish, this kind of set phrase is very common

So it is better to think of recurso de reposición as one complete technical term, not to translate each word separately.

Por si means in case or just in case.

It expresses a precaution: you do something now because something else might happen later.

So:

• Guardamos el recurso de reposición por si lo desestiman
• We keep the appeal in case they reject it

This is different from plain si, which just means if.

Compare:

• Si lo desestiman, actuaremos = If they reject it, we’ll act
• Lo guardamos por si lo desestiman = We keep it in case they reject it

The second one focuses on preparation/precaution.

Because por si is normally followed by the indicative, not the subjunctive.

Spanish treats por si as referring to a possible real situation that you are preparing for:

• por si viene = in case he/she comes
• por si llueve = in case it rains
• por si lo desestiman = in case they reject it

So desestiman is the expected form here.

Using desestimen after por si would sound non-standard in this structure.

Lo is a masculine singular direct object pronoun.

In isolation, the most natural thing it refers to is el recurso de reposición.

Why lo?

• recurso is masculine singular
• desestimar takes a direct object
• so the standard pronoun is lo

That is why you get:

• desestiman el recurso → lo desestiman

Not le, because le is normally for an indirect object (and in standard Spanish, not for things like this).

One small nuance: in a wider context, lo could refer to some other masculine singular noun already mentioned, but if you only look at this sentence, el recurso is the obvious reference.

The subject is not stated, which is very normal in Spanish.

The ending -an already tells you it is third person plural:

• desestiman = they reject / dismiss

In this legal context, they would usually mean something like:

• the administration
• the authority
• the court
• the decision-making body

Spanish often leaves subjects out when they are clear enough from the verb form or the context.

It is mostly formal, especially legal or administrative.

In Spain, desestimar un recurso is a very standard legal expression meaning:

• to dismiss an appeal
• to reject an appeal

In everyday speech, people might more often use verbs like:

• rechazar
• denegar
• no aceptar

So this sentence has a noticeably formal/legal register.

Because el points to a specific appeal, one that the speakers already have in mind.

So:

• el recurso de reposición = the appeal / that specific appeal
• un recurso de reposición = an appeal / some appeal

In legal or administrative contexts, the definite article is especially common when everyone involved knows which document or procedure is being discussed.