DDoS is usually carried out by something called bots or a group of computers sending requests to a specific server at the same time. DDoS attacks are considered very dangerous and could lead to the loss of thousands or sometimes even millions of dollars.
During a DDoS attack no one is gonna be able to access the network resources or if it were carried out on a web server, the customers or users of that company won’t be able to Carry out any transaction which invariably means a loss of cash for the company.
But of course there are several ways a person could prevent against DDoS attacks one of which is providing a very strong firewall to fight SYN flood attacks and also limiting the number of login attempts by a user.
Of cause there are other ways you could prevent yourself from such attacks but those are just the ones i think are most important.