A more thorough explanation:

Definition: A DoS attack, also known as a denial-of-service attack, is a malicious act that targets a computer, website, network, server, or database with the intention of making it inaccessible. This is usually done by overwhelming the system with activity or by forcing it to malfunction. It is also known as nuking. A distributed denial-of-service attack (DDoS attack) is carried out by distributing a virus that causes infected computers to try to access the target computer at the same time.

Examples:

• A hacker sends a large number of requests to a website, causing it to crash and become unavailable to users.
• A virus infects multiple computers and uses them to send a flood of traffic to a server, making it impossible for legitimate users to access it.

These examples illustrate how a DoS attack can render a system inaccessible by overwhelming it with activity or causing it to malfunction. In the first example, the website is targeted directly, while in the second example, the attack is distributed across multiple infected computers.