A more thorough explanation:

Definition: A negotiorum gestor is a person who acts without authority to protect another person's interests, in the reasonable belief that the owner would approve the action if made aware of the circumstances. This term comes from Roman and civil law.

For example, if a person sees a house on fire and enters it to save the owner's property, they are acting as a negotiorum gestor. They are not authorized to do so, but they are doing it out of concern for the owner's property. If the owner later approves of their actions, the negotiorum gestor can claim compensation for their trouble, and the owner can claim for any loss that results from the gestor's fault.

A negotiorum gestio is a quasi-contractual situation in which an actor (negotiorum gestor) manages or interferes in the business transaction of another person (dominus negotii) in that person's absence, without authority but out of concern or friendship. By such conduct, the actor is bound to conduct the matter to a conclusion and to deliver the transaction's proceeds to the person, who likewise must reimburse the actor for any expenses incurred. A negotiorum gestio does not exist if the gestor acted self-interestedly or if the owner expressly forbade the gestor from acting on the owner's behalf.

Overall, a negotiorum gestor is someone who acts in good faith to protect another person's interests, even if they do not have explicit authorization to do so. They can be compensated for their trouble, but they are also responsible for any mistakes or faults that occur as a result of their actions.