A more thorough explanation:

A personal treaty is an agreement between two nations or sovereigns that relates exclusively to the contracting sovereign as a person. Examples of personal treaties include family alliances and treaties guaranteeing the throne to a particular sovereign and their family. Personal treaties have lost their importance with the advent of constitutional government in Europe.

For example, a personal treaty could be an agreement between two monarchs to marry their children to each other, thereby creating a family alliance. This type of treaty would not necessarily involve the entire nation, but rather the personal interests of the monarchs and their families.