ELI5 National Debt - who do countries owe when they are in debt?

If every major country is in billions of debt, where are they borrowing from? Who exactly do they owe? Are there any consequences for them being in debt? Why is it a problem when it gets large? Why was Greece in trouble, while Japan seemingly has high debt but is fine?   ... We explain like i'm five!     Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: emils5, poomanshu, potato_octopi, cop-disliker69, reshkayden, agoraiosbum, cheesewithwhine, aythekay & anjabe96 To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected]