According to Apple:
Because you use your Apple ID for so many Apple products and services, you should make sure that your Apple ID is as secure as possible. You should be the only person who knows your password and can sign in with your Apple ID. If someone you don’t know or don’t trust can sign in with your Apple ID, your account is not secure.
Your Apple ID might not be secure for the following reasons:
- Someone else created an Apple ID on your behalf, or you’re using an Apple ID that was already signed in when you received your device.
- You’re sharing an Apple ID with family or friends. Your Apple ID is your personal account. If you want to share purchases with a family member, use Family Sharing. With Family Sharing, you can share a calendar, photos, reminders, and more without sharing your Apple ID.
- You don’t recognize the Apple ID that is signed in on your device.
- You shared your password with someone else intentionally or unintentionally. For example, someone else selected your password for you, you told someone your password, or you entered your password on a phishing site.
- You don't have control of the email address or phone number associated with your Apple ID.
- Your password is weak or is compromised.
- You share your devices with someone else, your devices are not protected by a passcode, or your passcode is easy to guess.
If any of the above are true, you should reset your password as soon as possible and review your account information.