Technical debt occurs when you take a shortcut, thinking "this will do for now. I'll sort it out properly later." And then you keep putting off "later", and probably forget about the issue ... until it comes back to bite you.

It's all those tiny things like a misspelled variable, whose misspelling has to be replicated evermore, or making a configuration change on a live site without capturing it in code, so that the live site is now different to the development one.

It's going [copy]-[paste] to a chunk of code when you really should be abstracting it into its own function.

It's neglecting coding standards. Even 'just this once', making your code harder to read and understand.

It's adding a */