～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.