A more thorough explanation:

A constitutor is a person who agrees to take responsibility for paying someone else's debt.

For example, if John owes $10,000 to a bank and cannot pay it back, he may ask his friend Jane to become his constitutor. If Jane agrees, she will be responsible for paying back the $10,000 to the bank if John cannot.

This concept comes from Roman law, where a constitutor was known as an "orderer" or "arranger." It was a way for people to help each other out when they were in financial trouble.