Approval process allows for notifications and control over your deployments through your Pipeline. Receive notifications on the deployments set to the stages of the Work Items. Set approvers where you can have a majority rule on the approval of the deployment.
Setup Approval Process in your instance.
Step 1) Ensure Work Items is enabled in your team settings. Set the pipeline you wish to have an approval process assigned too.
From your pipeline tab select the Pipeline you would like to set approvers (you can set a variation of the amount of approvers and what environments you want approval of the deployment).
Step 2) Create a Work Item and set the approval process check marks.
Note: In order to be an "approver" the user must have the owner role assigned
Step 3) Once you create a Work Item you will now need to select the approvers. An email will then be sent to the user selected.
Please view this video below for a walk through of the entire process: