An output descriptor is a type of notation that is used to describe the conditions under which a cryptocurrency output can be spent. It is a way of specifying the rules and requirements that must be met in order to spend a particular output, such as the amount of the output, the type of script that is used, and the type of key that is required. Output descriptors are commonly used in Bitcoin and other cryptocurrencies, and they are supported by many different wallet software and other tools. Output descriptors are a useful way of representing the rules and conditions that apply to a particular output, and they can be used to automate the process of generating and signing transactions.