system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What is the significance of debt to GDP ratios and how does it really affect economic growth or stability

I saw the US governments debt to GDP ratio is over 100%. Whats the significance of this, since apparently Japans ratio is over like 300% does this even matter or is it being used by politicians as a scare tactic. Whats the limit of this ratio to where the country might be in danger of collapse? 500%? 1000%? 

I saw some article talking about Indias "catastrophically" high ratio of 64% meanwhile Japan is cruising with 5 times that and is still an economic superpower.
gpt
An important factor is who the country owes money to. If the country owes money to entities outside of the country, a high debt to GDP ratio is ratio is potentially dangerous as their creditors will be unlikely to make concessions should the country hit hard times. However, if most of the debt is owed to domestic institutions, the debt is much more manageable since an economic downturn in the country would hurt the lenders as well, so they have an incentive to keep the country solvent and make concessions. This is how Japan is able to maintain such a high debt to GDP, it's creditors are almost all Japanese entities and so they won't act in a way that undermines the Japanese economy.

India on the other hand borrows mostly from entities outside of India and they are far less likely to be forgiving should India have trouble making debt payments. This means India would have a much rougher time should they need to do restructuring of their debt.

In general external debt like what India carries is much more risky than the internal debt that Japan has, and that's what allows Japan to carry a higher debt load.

Another factor is the interest that the creditors charge, a stronger economy can borrow at a better rate and that allows them to also have a higher debt load since the interest payments are lower.