When you’re developing on an instance you might want to change something in a database where the change might require to re-create the table. By default, the SQL Server Management Studio (SSMS) will prevent saving changes that require the table to be recreated.
Examples of changes that require table re-creation:
Change a column to no longer allow NULL values
Adding columns in the before another column
Moving a column
I agree with Sander: this is a useful feature, but not something you want to abuse. If you don’t understand the magnitude of your change, it could cause production problems. And if you do understand the magnitude of your change, typically you’ll want to script it out for later.