Technical Debt was a phrase originally used in software development to describe the cost of additional rework caused by choosing an easy (limited) solution now instead of using a better approach that would take longer. It has since become the common term to describe the same problem more broadly across the entire IT function.
Technical debt is often incurred through standard technology operations and projects through small decisions that seek to defer cost or effort and is largely invisible to the business until an appearingly small change in the business requirements causes an unexpected large project cost or delay.
It is considered best practice for organisations to maintain a technical debt register as well as have actively maintained lifecycle management plans for legacy systems and infrastructure, however this is not common.