max-width is a CSS property that sets the maximum width an element is allowed to reach. If the available space is wider than the max-width value, the element stops growing at that point. If the available space is narrower, the element respects the narrower constraint instead.
It's one of the most common sources of layout confusion on WordPress sites because themes and page builders set max-width on container elements, and those constraints propagate down to everything inside them. A section set to width: 100% doesn't mean 100% of the viewport. It means 100% of whatever parent container the section is sitting inside, which may itself be constrained to 960px or 1200px by a grandparent container several levels up.
This is why "make it full width" sometimes appears to do nothing: the element you're editing is already as wide as its container allows. The real constraint is on a parent element you haven't touched. Loupely Lens captures the full ancestor chain and surfaces max-width constraints on parent elements as part of the layout diagnosis.