A more thorough explanation:

Definition: Indebitatus assumpsit is a legal term that refers to a type of action in which the plaintiff alleges that the defendant contracted a debt and, as consideration, had undertaken (i.e., promised) to pay. It is a form of action in which the plaintiff seeks to recover damages for the defendant's breach of an implied promise to pay a debt to the plaintiff. The action was equivalent to the common-law action for debt (an action based on a sealed instrument), but could be used to enforce an oral debt.

Examples: If a person verbally agrees to pay a certain price for a certain parcel of goods and fails to perform, an action of debt lies against them. However, if they agree for no settled price, they are not liable to an action of debt, but a special action on the case, according to the nature of their contract. In this case, the plaintiff must recover the whole debt they claim, or nothing at all. For the debt is one single cause of action, fixed and determined; and which therefore, if the proof varies from the claim, cannot be looked upon as the same action of debt. But in an action on the case, on what is called an indebitatus assumpsit, which is not brought to compel a specific performance of the contract, but to recover damages for its non-performance, the implied assumpsit, and consequently the damages for the breach of it, are in their nature indeterminate; and will therefore adapt and proportion themselves to the truth of the case which shall be proved, without being confined to the precise demand stated in the declaration.

Explanation: Indebitatus assumpsit is a legal term that refers to a type of action in which the plaintiff seeks to recover damages for the defendant's breach of an implied promise to pay a debt to the plaintiff. The examples illustrate how the action works in practice and how it differs from other types of legal actions. It is important to note that the action is not brought to compel a specific performance of the contract, but to recover damages for its non-performance. The damages for the breach of the implied promise are in their nature indeterminate and will adapt and proportion themselves to the truth of the case which shall be proved, without being confined to the precise demand stated in the declaration.