Hierarchy is a term used in several contexts when dealing with computers. The general meaning is a 'tree', i.e. a data structure with a single 'root' node, from which descends (or ascends, depending on how it is represented) a branching set of nodes. A node (other than the root) has only one parent, but may have more than one child.
Most file systems are now hierarchies, an approach started by Multics. The Domain Name System of the Internet is also structured as a hierarchy, with nodes 'com', 'edu', 'gov' etc all childred of the root.
The term 'hierarchy' is also used in information systems architecture, where it is used to refer the the relationships between the sub-systems in a system.