Denial-of-Service (DoS) or in terms of "flood" hack, is a type of attack on a server in order to drastically reduce the ability to operate or even stop it. Attacking a web server can cause the server to crash and, as a result, all web pages supported by that server to malfunction.

The most common method of performing the Denial-of-Service attack is to send a huge number of requests to the server, which it is no longer able to process in a timely manner, which will lead to difficulties in server operation or even complete blocking.

You can see the effect on sites that suddenly become populated with a lot of users, and the server is not prepared for the large number of queries that users perform.