A Maria prefere congelar o pão em fatias, porque depois é fácil descongelar só o que precisa.

In European Portuguese, it is very common to use the definite article before a person's first name: a Maria, o João, and so on.

So A Maria does not mean the Maria in the English sense. It is just normal Portuguese usage, especially in Portugal.

You can sometimes hear or read Maria without the article, but A Maria sounds very natural in everyday European Portuguese.

Because preferir can be followed directly by an infinitive when the same person does both actions.

So:

• A Maria prefere congelar o pão = Maria prefers to freeze the bread

Maria is both the one who prefers and the one who freezes, so Portuguese simply uses:

• preferir + infinitive

You do not need a preposition here.

Because pão here is being used as a mass noun, like bread in English.

In English, you say:

• She freezes the bread
• not usually the breads

Portuguese works the same way here:

• o pão = bread in general, or the loaf/bread she has

Even though it will be cut into slices, the noun stays singular.

Em fatias literally means in slices.

So:

• congelar o pão em fatias = to freeze the bread sliced / in slices

It tells you the form the bread is in when it is frozen.

This is a very natural way to say it. In English we often use an adjective like sliced, but Portuguese often uses a phrase like em fatias.

• congelar = to freeze
• descongelar = to defrost / thaw

The prefix des- often gives the idea of reversing an action.

So:

• congelar = make something frozen
• descongelar = make it no longer frozen

This is similar to English pairs like do / undo.

Here porque means because.

The sentence has this structure:

• A Maria prefere... porque...
• Maria prefers... because...

So porque introduces the reason.

A quick contrast:

• porque = because
• porquê = the reason / why as a noun
• por que = why / for which reason in questions and some other structures

In this sentence, the correct form is porque because it gives an explanation.

Depois means afterwards, later, or then.

In this sentence, it refers to a later moment, after the bread has been frozen:

• porque depois é fácil... = because afterwards/later it is easy...

The idea is: Maria freezes the bread in slices now, and later it becomes easy to defrost only the amount she needs.

Portuguese does not need a dummy subject like English it in this kind of sentence.

English says:

• It is easy to defrost...

Portuguese says:

• É fácil descongelar...
• literally, Is easy to defrost...

This is completely normal in Portuguese. The structure is:

• é + adjective + infinitive

Examples:

• É fácil aprender. = It is easy to learn.
• É difícil perceber. = It is difficult to understand.

Here só means only or just.

So:

• descongelar só o que precisa = defrost only what she needs

In other contexts, só can also mean alone, but that is not the meaning here.

Here o que means what or that which.

So:

• só o que precisa = only what she needs

It refers to the amount of bread needed at that moment.

You can think of it as a shortened way of saying:

• only the part/amount that she needs

This use of o que is very common in Portuguese.

Because Portuguese often leaves out subject pronouns when the subject is clear from the context.

So instead of saying:

• só o que ela precisa

the sentence says:

• só o que precisa

The meaning is still understood as what she needs, with Maria as the implied subject.

This is very normal in Portuguese, and much more common than in English.

This is a very good question.

In European Portuguese, many teachers and grammars prefer precisar de:

• precisar de alguma coisa = to need something

Because of that, in very careful Portuguese you may also see:

• só do que precisa
• or more explicitly só daquilo de que precisa

However, in everyday language many speakers say:

• só o que precisa

with the preposition left out.

So for a learner, the safest point is:

• o que precisa is very common and widely understood
• but do que precisa or daquilo de que precisa may sound more careful or more traditional in European Portuguese

Yes, you can, but the structure is slightly different in feel.

• É fácil descongelar só o que precisa.
  = It is easy to defrost only what she needs.

This focuses on the action as a whole.

• É fácil de descongelar.
  = It is easy to defrost.

This often sounds more like a description of something.

In your sentence, é fácil descongelar só o que precisa is very natural, because the speaker is talking about the convenience of the action.