Trusted contacts are your friends in your facebook account that  you can reach out easily and you trust him/her. whenever you need thier help to get in to your facebook account.
For example

If you forget your account’s password u can easily get into your account with the help of your friend’s account. Or someone hacked your account and changed your account’s password You can get back your accounts by geting Codes from Your Friends.

After you set up trusted contacts, if next time you can’t get into your account your trusted contacts can access special, one-time security codes from Facebook via a URL. You can then call your friends to get the security codes and use those codes to access your account.

How do I add trusted contacts to my account?

Follow the steps to add Trusted accounts

1. Go to your Security Settings
2. Click on the Trusted Contacts section
3. Click Choose Trusted Contacts
4. Choose 3-5 friends and confirm your choices

How to Change and remove the trusted accounts from my accounts?

To change or edit the trusted friends account, follow steps 1 through 3 and click Edit next to your trusted contacts.

1. Go to your Security Settings
2. Click on the Trusted Contacts section
3. Click Choose Trusted Contacts

 Note; To Learn How to Use The Trusted Contact Click Here