"Technical debt" is a phrase developers use a lot without explaining what it actually means. Once you understand it, though, it becomes one of the more useful ideas for a business owner to have, because it explains why software projects that started out fast often get slower and more expensive over time.
Think of it like a loan
When a developer takes a shortcut to build something faster, that's the loan. You get speed now. But the shortcut usually means the code underneath is a little messier or less flexible than it should be. Later, when you want to change or add something nearby, it takes longer than it should, because that shortcut has to be worked around first. That extra time later is the interest on the loan.
An example you'll recognize
Say you're launching an online store and need it live in three weeks. Your developer hardcodes a single flat shipping rate directly into the checkout, rather than building a proper settings screen where you could manage rates yourself. It works fine, the store launches on time, customers check out without any issue.
Six months later, business is growing and you want to offer three different shipping rates for three regions. If that settings screen had been built from the start, this would be a five minute change you could make yourself. Instead, a developer has to go in, find where that rate is hardcoded, and build the flexible version that should have been there originally, then test that it hasn't broken checkout for existing customers. What should have taken minutes now takes days.
This isn't really about bad code
The shortcut made sense at the time. Getting the store live in three weeks was the right call. The problem isn't that a shortcut was taken, it's that nobody wrote down that it was taken, or planned for when it would need to be fixed. It just sat there quietly until it became a blocker.
How to tell it's becoming a real problem
Small changes that should be quick start taking noticeably longer. Your developer starts saying things like "that part of the code is a bit fragile, let's be careful" more often. New features seem to accidentally break things that were working fine before. If this sounds familiar, there's likely a build up of these shortcuts that's now actively slowing your team down.
What fixing it actually looks like
It rarely means starting over. It usually means your developer sets aside time, every few weeks or as part of each new feature, to clean up the specific shortcuts that are causing the most friction right now, rather than letting them all pile up and only dealing with it once everything feels slow.
The businesses that handle this well treat it as a normal, expected cost of building software, the same way you'd budget for maintaining equipment, rather than something to worry about only once it's already causing visible problems.