In your example, "#logo" is the "id" attribute of the <a> tag in the HTML. You can reference this ID by using #logo through your HTML and CSS. (Just as you would reference a particular class by using ".class")

Using "a logo" instead, or "a #logo" would not work. Another way to reference this particular link without using the id attribute would be to use it with its parent element, for example:

article a {
  color: #fff;
}

That would select the <a> tag (s) contained in the <article> parent element. Using the ID just makes things easier, since each id must be unique; you can find more info here.

Hope it helps!

in logo is used for ID, we use id and in future we will also use classes which helps to oragnize your css in better way, eg you have five paragraph in your html document but you want to style a particular paragraph in different way ID is helpful, classes and id's can be used for a lot of things, so maybe you have same navigation design across all your website pages (eg index.html, about.html, contact.html) you can target all of them with class or id

Also logo is not a keyword in css so you cant directly use it in css, you can use specific keywords i.e tags name (eg p, img, ul, li, a, header, nav) to style

a is a reserved word for a link& logo is just a name to use to refeence something.