A CSS selector is the part of a CSS ruleset that actually selects the content you want to style.
head will select the element with the head tag
.blue selects all elements with the ‘bed’ class
#menu selects the elements with the ‘menu’ Id
div.row selects all elements with the div tag and the ‘row’ class
[aria-hidden="true"] selects all elements with the aria-hidden attribute with a value of “true”