Good morning, @NicholleB ...
External tools are third-party LTI (Learning Tools Interoperability) apps that integrate with Canvas. If someone from another school shared content to Canvas Commons that was using a LTI app but you do not have that app integrated into your own Canvas environment, that app (external tool) will not work. Also, it's usually the case that LTI external tools require a unique Consumer Key and Shared Secret (similar to a Username and Password) during the setup/configuration process. So, I'm not sure that the external app would work anyway even if both your school and the school of the person who shared the content had the same integration in Canvas.
I hope this helps a bit. Please let Community members know if you have any other questions about this...thanks! Take care, stay safe, and be well.
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