Learn Romanian: Nu, ușa este acolo. - No, the door is there.

In Romanian, the word for "the" attaches to the end of the noun rather than sitting in front of it. For feminine words ending in -ă, you replace that -ă with an -a to mean "the". This is why o ușă (a door) becomes ușa (the door).

I remember learning e for "is". Why does this sentence use este?

este and e both mean "is" and come from the verb a fi (to be). e is just a shorter, more everyday version of este. You could perfectly say Nu, ușa e acolo and it would mean exactly the same thing.

Is acolo related to aici?

Yes! They are a pair of location words. acolo means "there" (pointing at something further away), while aici means "here" (close to you).