Denial-of-service or distributed denial-of-service” attack. An attempt to make a machine or network resource unavailable to its intended users. DoS threats usually target websites or services hosted on high-profile web servers like banks and credit card payment gateways, but they’re also common in business in the form of website attacks.
For example, one common type of attack on businesses would be attempting a service overload — i.e. flooding a network with so much information that it either can’t respond to legitimate traffic, or responds so slowly that it’s basically useless.
Sadly, there isn’t much you can do to avoid being a victim of a DoS attack except securing passwords and installing anti-virus software and a firewall. Signs of an attack are unusually slow network performance, unavailability of a website, or the inability to access a website. Contact your IT team if you sense something’s off (Source).