When you create a subsite on a SharePoint site, many times you want to allow only certain people to have access to that subsite. You can set those permissions so that they are unique to that subsite.
1. Select the subsite in your browser.
2. Click the gear icon in the upper right-hand corner and then click on Site settings.
3. Select Site permissions.
4. If the page says "This Web site interits permissions from its parent", then click the Stop Inheriting Permissions button.
5. You'll be presented with a screen to assign groups. For each of the three categories, select Create a new group as shown below and click OK.
6. Return back to the Site permissions page (gear icon > Site settings > Site permissions). Now the site says that it has unique permissons. Remove the users and groups that you don't want to have access to the subsite by putting a check in the box in front of their name and click on the Remove User Permissons icon.
7. You can add new users by clicking the Grant Permissions button.