In a technical sense it refers to the secondary scriptures like the Manusmṛti which remind one, of the great spiritual truths contained in the Śruti or the Vedas. See DHARMAŚĀSTRAS for details.
In a more literal or common sense it means memory or remembrance.
After directly perceiving or experiencing any object of sense through the jñānendriyas or the five organs of know-ledge, when a person remembers that experience, it is called smṛti. In such a smṛti, either all the experience is remembered or a little less but never more. This is how it has been defined by the Yogasūtras (1.11). It is one of the five kinds of mental modifications (cittavṛttis).