This means your site needs to load these external resources before your visitors can see your actual content.
In practice, this causes visitors to miss important features or even leave your site, increasing your bounce rate.
Plus, this may cause search engine crawlers/bots and SEO tools to have difficulty properly reading your site’s content.
For stylesheet files, you may consider moving the relevant parts into inline CSS. However, you should not use inline CSS unless you have to. Codeacademy gives some good advice on when to use and when not to use inline styles.