An inline style is a CSS declaration written directly on an HTML element's style attribute, rather than in a separate stylesheet. It looks like this: <div style="color: red; font-size: 16px;">.
Inline styles sit at the top of the specificity hierarchy. They beat almost every rule written in a stylesheet, regardless of how specific that stylesheet rule is. The only thing that reliably overrides an inline style is a stylesheet rule marked with !important.
The problem for site owners is that inline styles are often applied by JavaScript at runtime, meaning they're not in any stylesheet you can find and edit. Page builders set them when you adjust element-specific settings. Plugins inject them at page load to control positioning, sizing, or visibility. When Loupely Lens identifies that a winning rule is an inline style, it traces the source: if a page builder or plugin is setting it, the diagnosis routes to the appropriate fix in that tool's interface. If JavaScript is setting it at runtime, the diagnosis routes to developer handoff, because no CSS rule you can paste in will reliably beat JavaScript-injected inline styles.