A Requirement specifies a capability or a condition that must (or should) be satisfied. Requirements are used to establish a contract between the customer (or other stakeholders) and those responsible for designing and implementing the system. A requirement can also appear on other diagrams to show its relationship to other modeling elements.
When a requirement nests other requirements, all the nested requirements apply as part of the container requirement (the requirement that contains all the nested requirements). Deleting the container requirement will thus delete all the nested requirements it contains; a functionality inherited from UML.