There are times when people would be pressing to open one program several times when the program can only be opened one at a time. This is the critical period because the OS does not allow this to occur. Not all of the regions in the operating system are like this but it will help if you would know which ones. This will prevent you from pressing far too many times.
There are instances when doing this would cause the program to crash for some time and it would need to be rebooted so that it can work again. Take note that there are also some critical regions that are locked to those who are unauthorized to view the content.
It is a piece of code which only one process executes at a time.
When there is a concurrent access by many processes to a single resource, then it may lead to unexpected behavior by the operating system. Hence there are some regions in the operatingsystem which can be accessed by only one process at a single time, those type of regions are called "critical regions".