A CSS selector is the part of a CSS rule that identifies which elements on the page the rule applies to. It comes before the curly braces that contain the actual styling instructions.
Selectors range from broad to precise. A type selector (button) matches every button on the page. A class selector (.primary-button) matches every element with that class. An ID selector (#submit-button) matches only the 1 element with that specific ID. Selectors can be combined: .header .nav-link matches only elements with the class nav-link that are inside an element with the class header.
The selector is what determines a rule's specificity score, which determines whether that rule wins when it conflicts with another rule trying to set the same property. Understanding the selector is usually the first step to understanding why a style you set isn't taking effect. When Loupely Lens identifies a winning rule in Fight View, it shows you the full selector so you can see exactly what the rule is targeting and why it scored higher than yours.