When you access a website, the computer will perform DNS caching, which means that the data in the form of domain name and IP address of the website that you access are locally stored in the computer for a period of time, so when you open a web page that has previously been opened it will feel faster because the computer has remembered the IP address and it does not need to contact the DNS server. However, which often be a problem is that the DNS cache can sometimes be corrupted and it causes problems like some websites that can not be accessed when the internet connection has no problem. At this point the DNS flush is needed.
DNS flush will force the computer to clear the DNS cache or in other words it forces the computer to erase all memory of the domain name and the IP address associated with the domain name. Once the DNS cache is empty, the computer will start asking for new data from the DNS server and it will build DNS cache from the beginning.