Mixed content needs to be fixed, because only when your site loads all content and resources over HTTPS, a client would see a green padlock near your domain name in a browser address bar.
Green padlock indicates that your site content is 100% secure.
You can find insecure items links in the Developers Console of your browser. The console could be found with pressing one of the following keyboard shortcuts in your browser:
Ctrl + Shift + J (Windows/Linux)
Fn + F12 (Windows/Linux laptops)
Command + Option + J (Mac)
Command + Option + C (Mac)
The most common issue is insecure images. You can remove them in your site's WordPress admin area → Media → Library section and then import them again. This will help to make your site 100% secure. Alternatively, you may use one of these free helpful plugins to fix the mixed content issue: