This means that firms are asking you to confirm that you are still considering their offer. Offer letters may require that you reaffirm, or else your offer will expire, so be sure to read them carefully for this requirement. Other times, firms may reach out informally if they have not heard from you to ask for a reaffirmation.
The reason or this is that most firms are managing their offers based on their expected class size, and may extend offers over time. They are carefully tracking their offers so that they can extend offers to additional candidates once they know the outcomes of their current set of offers. If you are no longer interested in an offer, please release it as soon as possible. If you are still considering an offer, it is helpful to the firm to let them know that as well.