The cascade is the algorithm browsers use to decide which CSS rule wins when multiple rules try to set the same property on the same element. Every time you change a color in your page builder and the page ignores you, the cascade is making that decision without you in the room.
It works in 3 stages. First, the browser looks at where each rule came from: inline styles beat stylesheet rules, and stylesheet rules beat browser defaults. Second, among rules from the same origin, it compares specificity: how precisely each rule targets the element. A rule targeting a specific ID beats a rule targeting a class, which beats a rule targeting an element type. Third, if 2 rules are still tied, the one that appears later in the code wins.
The reason visual problems take hours to fix isn't that the information doesn't exist. The browser has already resolved the cascade completely by the time the page loads. The answer is sitting there. The problem is that reading it requires knowing where to look and what the output means. Loupely Lens reads the cascade for you and translates the result into what's actually happening and what to do about it.