あく

to be opened (e.g. doors)

This is the intransitive form of the verb, i.e. it does not take an object. The subject is doing the verb. In this case, the subject is open. ドアが開いています　「ドアがあいています」 "The door is open." This is opposed to the transitive verb form, i.e. the verb MUST take an object. The subject is doing the action TO or WITH the object. ドアを開けています　「ドアをあけています」 "I am opening the door." (Note that Japanese does not require the subject to be specified when it is understood/implied by context.) by Shimapanda

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