Hello!
We're talking about "overhangs" here - that's a situation where a part rapidly increases it's width or cross-section as we move up the part. You see the simpler 3D printers work in layers as they deposit molten plastic to make a part. If the next layer is significantly bigger than the one underneath it, you're starting to deposit the plastic on air, where it obviously cannot stay and it just drips. You can print things like vases or statues, where the cross-section increases gradually, but you would run into problems if you tried to print, say a table - unless you chose to print it upside down - that's where the design begins. I hope that clears it up - have a nice day
Paweł