In both English and French, certain places are considered to have a kind of general version as well as a specific location.
Here are a couple in English:
Where did you get that book?
Oh, the library.
It doesn't really matter which library or which branch for you to use this sentence. You wouldn't say "a library" here.
You're a fan of summer?
Oh, yeah. I love the beach.
Which beach, you might wonder? The answer would probably be all beaches. :) You don't say you love "a beach".
And the closest example to the one you asked about is probably user168676's "the movies", which I think is also a clever way to mirror the ambiguity of cinéma (the building or the show?).
There are finer differences between how English and French use le and un, but as you begin your journey of learning French, I wouldn't worry about them too much yet (including the last line of user168676's answer). For now, it's probably enough to see that the two languages are actually pretty similar in this case.