What is: Spam
Spam is a known word for unwanted messages or content. It includes unwanted or harmful emails, pop-ups, online posts, illegal links etc. Spam can question the integrity of a website. It is very important to stay safe from these unwanted content/messages. Thus, there are various WordPress plugins available to prevent your site from Spamming. These Plugins are also called Anti Spam WordPress Plugins.
People use harmful messages and content with various intentions such as Stealing money, installing viruses on computers, boosting search engine ranking (by including too many backlinks), stealing personal information. Use firewall or malware protection Plugins as well as anti-virus in your computer to avoid such messages.
People make wrong comments to increase backlinks as well as website traffic. This has been increasing for a long time but now Search Engines such as Google have defined these Comments as black hat SEO.
To combat from such comments, you can turn on Comment Moderation in the WordPress, so that you can approve the real comments and move the spam comments into trash. You can also use the anti spam protection plugins to avoid such comments.