Inline CSS refers to the practice of including CSS (Cascading Style Sheets) styles directly within the HTML elements of a webpage, rather than in a separate external stylesheet. This means that instead of defining styles in a separate .css file, you write them directly within the HTML tags using the style attribute.
<p style="color: blue; font-size: 16px;">This is a paragraph with inline CSS.</p>
The issue with inline CSS is that the browser has to render the actual CSS file and the inline CSS information which could be transferred to the CSS file. It does not slow down your site significantly, but It's a good issue not to have.
In order to fix code related issue we would recommend to contact an expert Shopify theme developers, example: https://experts.shopify.com/