Technical debt is the incomplete work that must be addressed to bring the product to the expected quality and functionality.
Technical Debt could be intentional as well as unintentional.
Intentional Tech Debt is a result of decisions made during product development that emphasize speed over the product quality.
When developers are rushed, they may cut corners and pay less attention to quality - the resulting product may have incomplete work / activities.
For example: lack of refactoring, not fixing known issues / defects, incomplete or ignored unit testing, etc.
Unintentional Tech Debt is a result of slopiness, lack of technical competence, bad design decisions, etc.
Having Technical Debt lowers the User Experience and also increase the cost of ownership.
Also see: Living Without Tech Debt