Users may find that SharePoint keeps asking them for their login. If SharePoint giving users a login popup, then either the SharePoint website isn’t added to the Trusted Sites list.
To fix this and stop the login prompt from popping up, add SharePoint to the Trusted sites list in Internet Explorer (Internet Options -> Security tab -> Select Trusted Site and click Sites). Next set Trusted sites, click Custom Level, scroll right to the bottom and under User Authentication, Login, select Automatic logon with current user name and password. You can manage this with GPO is the issue is organisation wide.
If SharePoint persistently gives users a logon popup despite being set correctly, it may be that incorrect credentials are saved and being presented to SharePoint. Typically this can occur if users passwords are set to expire. This behaviour may be supported by users AD accounts being locked out. This is further evidence that saved credentials are causing SharePoint to keep popping up a login prompt.
To fix this, go to the Credential Manager in the Control Panel and remove the Windows Credentials for the SharePoint site.