Territory management is an account sharing system that grants access to accounts based on the characteristics of the accounts. It enables your company to structure your Salesforce data and users the same way you structure your sales territories.
To Enable Territory Management
Step 1 : Go to Setup
Step 2 : Enter Territories in Quick find box
Step 3 : Select Settings under Manage Territories
Step 4 : Click Enable Enterprise Territory Management
Enterprise Territory Management has been successfully enabled for your organization. From this settings page you can configure access rules for accounts and opportunities associated with territories.
This blog is very useful to learn about territory management activation.