Hidden text in the SEO term is a method of placing text and links in HTML invisible to the user and making it visible only to search engine robots, and is one of the representative search engine spams.
There are various things to hide even if it says hidden “text”, and in addition to normal text, images and links are also included. There used to be spam that tries to deceive search engines by hiding links and creating large numbers of links.
The hidden text will be invisible to the user , for example, in the shape of white letters on a white background, or by setting CSS as text-indent: -9999 and skipping the target off the screen. The problem was that the search engine crawlers would get the data properly.
In recent years, especially google has come to look rather strict, and such methods have become inapplicable, but it has been reduced, but it is still used even if you do not intend to spam, such as replacing images, so be careful. is.