Defined by Foote & Yoder (1997), it describes software projects that have grown in a chaotic fashion (which is what happens when you have no design, see Good design vs Bad design).
What we usually end up with is a system without clear boundaries and murky interactions. Changing things in it is a dangerous and mind-boggling task. It's not rare to see a change cause a chain reaction (which might surface weird bugs).
Preemptive design and solid project concepts are good tactics against this issue. Strategic Design around Domain-Driven Design (DDD) is a popular approach to that.