What is the mixed content issue?
Secure websites using an SSL certificate starts with https:// in the address URL. Having an SSL certificate allows your web browser to communicate securely to the site's webserver. That means whatever information transferred between your web browser and the webserver is encrypted and therefore cannot be hacked or infiltrated. This is particularly important if you're sending sensitive information such as personal details, banking details, and credit card payment details.
Image source: SSL2buy
While technically not a ranking factor, seeing this warning message can deter users from fully interacting with your website. Especially if your webpage provides transactional support for online purchases such as credit card forms and personal information capturing forms.
Use this tool to check if any of your web pages have mixed content.