Es macht einen Unterschied, ob die Röstzeit lang ist.

Here, Es is a grammatical subject. German often uses es in expressions like Es macht einen Unterschied just as English uses it in It makes a difference.

So the es does not refer to a specific thing. It is there because the sentence needs a subject.

A very close structural translation is:

• Es macht einen Unterschied = It makes a difference

Because Unterschied is the direct object of machen, so it must be in the accusative case.

• der Unterschied = nominative
• einen Unterschied = accusative

Since Unterschied is masculine, the article changes:

• ein Unterschied
• einen Unterschied

This is part of the fixed expression einen Unterschied machen = to make a difference.

Yes. Einen Unterschied machen is a very common German expression meaning to make a difference.

Examples:

• Das macht keinen Unterschied. = That makes no difference.
• Das kann einen großen Unterschied machen. = That can make a big difference.

So in your sentence, macht einen Unterschied should be understood as one unit of meaning.

Ob means whether.

It introduces a clause about something uncertain, something that can be true or not true. In this sentence:

• ob die Röstzeit lang ist = whether the roasting time is long

A useful way to think about it:

• ob is used for whether / if in the sense of uncertainty
• not for a time condition like when

This is a very common question.

• ob = whether
• wenn = if / when in a conditional or repeated-time sense
• falls = if in a more clearly conditional sense

So:

• Es macht einen Unterschied, ob die Röstzeit lang ist. = It makes a difference whether the roasting time is long.

This means the important issue is: Is it long or not?

But:

• Es macht einen Unterschied, wenn die Röstzeit lang ist.

would mean something more like:

• It makes a difference when / if the roasting time is long.

That is a different idea and usually not what is meant here.

Because ob die Röstzeit lang ist is a subordinate clause.

In German, subordinate clauses send the conjugated verb to the end.

Compare:

• Main clause: Die Röstzeit ist lang.
• Subordinate clause: ..., ob die Röstzeit lang ist.

This is one of the most important word-order rules in German.

Because in German, a subordinate clause is normally separated from the main clause by a comma.

So:

• Es macht einen Unterschied, ob die Röstzeit lang ist.

The comma marks the beginning of the ob-clause.

Because Röstzeit is a feminine noun:

• die Röstzeit

Here it is the subject of the subordinate clause, so it stays in the nominative:

• die Röstzeit ist lang

It is a compound noun:

• rösten = to roast
• die Zeit = time

So die Röstzeit literally means roasting time.

German often builds precise meanings by combining nouns like this.

Because lang is used here as a predicate adjective after sein.

In German, adjectives used after verbs like sein, werden, and bleiben are not inflected.

So:

• Die Röstzeit ist lang.
• not Die Röstzeit ist lange.

Compare:

• eine lange Röstzeit → adjective before a noun, so it gets an ending
• die Röstzeit ist lang → adjective after sein, so no ending

Here it is best understood as a predicate adjective.

It describes die Röstzeit:

• die Röstzeit = the roasting time
• ist lang = is long

So it behaves like an adjective, even though in English we might sometimes think of time expressions a little differently.

Usually, in this exact structure, Es is the normal and natural choice:

• Es macht einen Unterschied, ob die Röstzeit lang ist.

You could also restructure the sentence, for example:

• Ob die Röstzeit lang ist, macht einen Unterschied.

This is grammatically possible, but it is less natural in many contexts and sounds more marked or stylistic.

So for everyday use, the version with Es is the one learners should remember first.

Yes, very much. You can reuse the same structure with many nouns and clauses:

• Es macht einen Unterschied, ob der Kaffee frisch ist.
• Es macht einen Unterschied, ob das Wasser heiß genug ist.
• Es macht einen Unterschied, ob man Arabica oder Robusta verwendet.

So a useful pattern is:

• Es macht einen Unterschied, ob ...
• It makes a difference whether ...

This is a very practical sentence frame to learn.