Learn Swedish: De spelar fotboll.

In everyday spoken Swedish, the word De (they) is almost always pronounced as "dom". It is a very common pronunciation rule in the language, so whenever you see De, just remember to say "dom"!

I see spelar ends in "-ar". What does that tell me about the verb?

It tells you that spelar is in the present tense and belongs to Group 1! Group 1 is the most common category of regular verbs in Swedish, and they all take the -ar ending in the present tense. Its base form (the infinitive) is spela.

Could this sentence also mean "They are playing football"?

Yes, absolutely! Swedish does not have a separate "is/are doing" form (the present continuous) like English does. Depending on the context, De spelar fotboll translates to both "They play football" (in general) and "They are playing football" (right now).

Why isn't there an en or ett before fotboll?

Just like in English, we usually do not use an indefinite article when talking about playing a sport. You say "They play football", not "They play a football". Because of this, we do not need en or ett here; we just say spelar fotboll.