Das Line-up nennt die Vorband.

Because Line-up is a borrowed noun in German, and it is usually treated as neuter, so it takes das in the singular.

In this sentence, das Line-up is also the subject of the sentence, so das is the nominative article.

• das Line-up = the line-up

Yes. In music, festivals, concerts, and similar contexts, Line-up is a common borrowed word in German. It refers to the list of performers or the set of artists appearing at an event.

It is usually written Line-up with a hyphen.

You may also see the plural:

• die Line-ups

nennt is the 3rd person singular present tense of nennen.

The verb nennen often means:

• to name
• to mention
• to state
• to list

In this sentence, it means something like mentions or lists, not is called.

For example:

• Er nennt einen Namen. = He names/mentions a name.
• Das Programm nennt alle Künstler. = The program lists all the artists.

Vorband is a feminine noun, so its article is die.

It comes from:

• vor- = before
• die Band = band

So die Vorband means the band that plays before the main act: the opening band or support band.

Also, nennen takes a direct object, so die Vorband is in the accusative case here. But for feminine singular nouns, the article is still die in both nominative and accusative, so the form does not change.

German word order helps here:

• Das Line-up = subject
• nennt = verb
• die Vorband = direct object

This is a normal subject–verb–object sentence.

Also, the meaning of the verb helps:

• something names / mentions / lists
• something else is named / mentioned / listed

So the line-up is doing the action, and the opening band is receiving it.

No, they are different.

• nennen = to name / to call / to mention
• heißen = to be called / to have the name

Compare:

• Das Line-up nennt die Vorband. = The line-up mentions/lists the opening band.
• Die Vorband heißt ... = The opening band is called ...

So nennt describes an action performed by the subject, while heißt describes the name something has.

Because this is a main clause in German, and in a main clause the finite verb normally comes in second position.

So:

• Das Line-up = first element
• nennt = second position
• die Vorband = the rest

That is standard German main-clause word order.

Yes. Both are singular in this sentence:

• das Line-up = one line-up
• die Vorband = one opening band

Nothing in the sentence suggests plural.

If they were plural, the forms would look different, for example:

• Die Line-ups nennen die Vorbands would be very unusual in real usage, but grammatically it would show plural forms.

In normal German, you would usually only see the singular here unless the context clearly talks about multiple events or multiple opening acts.