What does <LINK rel = "alternate" type = "text / html" href = ""> do?
The meaning of each part according to w3c:
rel : a set of space-separated tokens. Defines a list of tokens that define the relationship between the document containing the link and the destination specified by the link. Two categories of links can be created using the link element - Links to external resources and hyperlinks.
alternate : a type of hyperlink that gives alternative representations of the current document.
type : A valid MIME type for assigning a hyperlink. Gives the MIME type of the associated resource.
Thus, basically the tag provides a link to an alternative location for your document, which may be in another language. In your case, what the link does is that it shows the same site with slight changes, probably for people of a different geographic location (I'm not sure about that, but yes, this is a modified version of the same site.)
PS: Avoid placing links to sites in your questions, instead try entering a code or a violin. The problem is that most websites undergo changes over time, so the question becomes redundant for future links.
The rel=alternate link informs the user (search engines) that there is a different URL with the same content, but in a different form (e.g. language, mobile, ...).
Source: MDN Link Types