Complex sentence: In concurrent programming, a critical section is a part of a multi-process program that may not be concurrently executed by more than one of the program's processes/threads; in other words, it is a piece of program that requires mutual exclusion of access.
Split into simpler sentences: In concurrent programming, a critical section is a part of a multi-process program that may not be concurrently executed by more than one of the program's processes. In other words, it is a piece of a program that requires mutual exclusion of access.