Please read this, it explains every thing, I have made font weight bold to the part which specifically talks about your question.

When you define approval steps, you can assign approval requests to different users. One option is to use a user field to automatically route these requests. The user field can be any custom hierarchical relationship field such as Account Manager or the Managerstandard user field. To see some examples of specifying a user field as an approver field, see Examples of Approval Processes.

When a record is in the approval process, it is always locked; only an administrator can edit it. However, you can also allow the currently-assigned approver to edit the record. You can still add campaign members to campaigns locked for approval.

1. From the Next Automated Approver Determined By picklist, select a user field if you want Salesforce to automatically assign approval requests to an approver based on the value in the user field. For example, you may want approval requests automatically routed to a user's manager as specified in the Manager standard user hierarchy field. Or, you may want approval requests automatically routed to an associate specified in a custom Account Manager user field.
   • To specify a user to approve all approval requests, leave this field blank. You can select a user, or users, as the approver when creating an approval step.Warning
     If you leave the Next Automated Approver Determined By field blank, you can't automatically assign approval requests to the manager in any step you create for this approval process.
   • Optionally, select the Use Approver Field of Record Owner checkbox if you want the approval process to use theManager standard user field or a custom field on the record owner's user record instead of the submitter's user record. Once you use this setting in a step, and in subsequent steps, it refers to the manager of the approver, not the manager of the record owner.
   Note
   Make sure the assigned approver has access to read the records for the approval requests. For example, a user who does not have read access to an Expenses custom object will not be able to view expense approval requests.
2. Select Administrators ONLY... if you want only users with the “Modify All” object-level permission for the given object, or the “Modify All Data” permission, to be able to edit records that are pending approval. Select Administrators OR... if you want the assigned approver to be able to edit the records too. The assigned approver must also have edit access to the record through both their permissions and the organization-wide sharing defaults for the given object. When a record is submitted for approval, it is automatically locked to prevent other users from editing it during the approval process.
3. Click Next. If you are editing an existing approval process and don't want to make further changes, click Save to store your changes.