An application (like an email or chat client) or a mobile device recently tried to sign in, but couldn't verify it was really you. To prevent unauthorized access to your account, Google notifies you via email and/or SMS about this type of suspicious sign-in attempt.
How to help secure your account
Check your account's security information and activity to make sure no one besides you has attempted to sign in to your account.
If a sign-in, app, or device looks suspicious to you, click Secure your account and follow the instructions to help re-secure your account.
If you don't want to receive these notifications on your phone, change your security notification settings.