〜てある

You can use the te-form of a verb + the helping verb ある to characterize a situation that has been brought about on purpose by somebody who remains unnnamed in the sentence.

さむいので、ストーブがつけてあります。

The heater is on, because it is cold.

You can say 〜てあります if somebody, possibly yourself, preformed an action on purpose earlier which can be described in term of 〜ておきましたand if the result of that action can still be observed at this moment. Note that 〜てあります describes a current state, hence the present tense.