Strikethrough is one of the font decoration elements that provide the crossed-out appearance of the text.
In some medieval manuscripts "strikethrough" of text with red ink often functioned as a highlight, which is similar to modern underline. In modern usage, strikethrough can be helpful to mark up the deleted information or some old version search results for instance. Designers can also use the strikethrough to indicate the text that is no longer valid in a document or on a web page.
Sometimes strikethrough can be applied to show deactivated items in a list.
Double strikethrough is not commonly used but can be applied to indicate the changes in wording in legal documents.