"He who is guarantor for a stranger will surely suffer for it, but he who hates being a guarantor is secure"
The word guarantor is the phrase to strike hands with. It means that the person has made an arrangement or a deal. In this case it means that they have said that they will vouch for those they do not know. They have made a business arrangement to cover the debts of someone who is not family. This is not wise.
It is bad enough when you become the guarantor for those who are family, but those who make these kinds of deals for strangers are really jumping in the deep end.
Solomon is really saying that we should not tie our future to the performance of another through debt. You need to avoid debt and especially debt that seeks to predict the actions of others.
Debt threatens your security. It presupposes that you know what will take place in the future. It is this “I know what is going to happen in the future” that is poor judgment.