A figure so huge it is almost meaningless.
Trading Economics suggests 9,685,501,000,000 euros as the total of the sovereign debt of the eurozone nations. In round figures that is ten thousand trillion.

It's something like E25,000 for every person in the eurozone.
Sovereign debts are high as a proportion of GDP in the eurozone and of course in many other nations. All the time nations can service the debts, it is not too terrible.
The problems come if the markets in effect call time on the debts. There are concerns about the legal basis of part of the sovereign debt of eurozone nations (the part held in UK, which is a big chunk) in the event of a no deal Brexit. I don't understand the issue. It may be that it is all spin around the Brexit story, but the idea seems to be that the debts held in the UK assume that the UK is an EU nation, and if it isn't then the legal framework fails and they have to be moved. The problem is that there is no appetite for buying up great chunks of eurozone debt, so if the debt has to be moved from the UK its not clear where else it could go. The risk is that the markets would call time on these debts.
Surely there must be some other interpretation. However we have to hope that specialists do look at it. If it isn't an issue, great.