Answer:
open source software
Step-by-step explanation:
An open source software is any software that is released under an open source license, meaning that anybody can access its source code, see how it was made, and as the question says, "sutdy, change and improve it".
This is opposite to proprietary or closed source software where you can have a license to use the software but you can't see or change it's "building blocks" (i.e. code)