what is mySecureStorage for storing keys

[imperialrogers]

var api = xmtp.Api.create();
var client = await Client.createFromWallet(api, wallet);
await mySecureStorage.save(client.keys.writeToBuffer());

The above snippet is from official docs of xmtp. What is mySecureStorage for the above snippet ? Is it the instance of package flutter_secure_storage or any other storage method.