Technical debt means how much work it takes to update legacy solution to a modern solution. E.g. each time a new C++ standard is used, all code written with the old standard should be checked. The work time needed to do this is paying up the technical dept.

Now, if you are lazy, and didn't clean up the code, used the easy and sloppy solution, next time you have twice the work to be done. So the dept gets worse, if you do nothing.