"det er" is the equivalent of the English phrase "there is". (Literally, "it is") The confusion arises because in the English phrase "there" has lost its sense of indicating place and become a grammatical marker. E.g. in the sentence "there is a time to work and a time to play " time isn't anywhere. However, we still use "there" by itself to indicate place or position and it looks like the same word.
Cf: "det er mange stolen" lit = it is many chairs.
I hope this isn't too confusing!