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The first one would be the <title> element which should appear as the first entry after your <head>.
The second would be your meta description tag which is typically 160/180 characters in length and is used to describe the topic of the page it is on.
The third would be your meta keywords tag which is typically a comma separated list of words and phrases which describe the page. Not well supported these days but still recommended if you have the time to set one up. Usually takes no more than a minute or two if you've planned properly.
Those are the primary areas that many refer to as hidden or not shown on your actual web page but served to the search engine in the metadata behind the page. Your title is normally shown as the link to your website in the serps (search engine results pages). Your meta description may or may not be used as a description of your site right underneath that linked title.