Comprei um croissant para a viagem, mas acabei por o guardar para depois do almoço.

Portuguese often leaves out subject pronouns when the verb ending already makes the subject clear.
Here, comprei and acabei both clearly mean I bought and I ended up. You could add eu for emphasis, but it is not necessary.

Both are in the pretérito perfeito simples, the normal tense for completed actions in the past.
So comprei = I bought, and acabei = I ended up / I finished in a completed past situation. In English, this is usually translated with the simple past.

Acabar por + infinitive is a very common way to say to end up doing something.
So acabei por o guardar means I ended up keeping/saving it. It often suggests that the final result was different from the original plan.

O is the direct object pronoun meaning it, referring back to um croissant.
So o guardar literally means to keep it / to save it. English puts the pronoun after the verb here, but Portuguese can place it differently.

Because o is the standard direct object pronoun.
In this sentence, the croissant is the thing being kept, so Portuguese uses o = it. Ele is mainly a subject pronoun, like he or it, not the normal direct object form in standard European Portuguese.

Here guardar means to keep, to save, or to set aside.
It does not mean to guard in the English sense of protecting something. With food, guardar para depois often means to save for later.

Because para shows purpose or intended use.
Comprei um croissant para a viagem means I bought a croissant for the trip — in other words, to take with me on the trip.
If you said na viagem, that would mean on/during the trip, which is a different idea.

It means the croissant was being saved for the time after lunch.
In natural English, you might translate it as for after lunch, until after lunch, or to have after lunch, depending on style. In Portuguese, guardar para depois is a very normal structure.

Yes, croissant is normally treated as a masculine noun in Portuguese, so you say um croissant.
Borrowed words are often masculine by default, and this is the standard form you should learn.