You have made a migration of your WordPress from http to https but some links and images still have “http” into their URL ?
When you migrate your website from an URL to another URL or simply change http to https, some URLs may not be changed. Why ? There are lots of explanations: your page & post content have not been changed, you use a custom plugin or a custom shortcode etc.
The goal is to replace every URLs, you can do it manually of course but it may be a pain when there are a lot of URLs !
We highly recommend to use a search & replace plugin.
The use of this plugin is pretty simple. You have to search for
http://www.yoururl.com and replace with
https://www. in all tables.
If this plugin is not enough to solve your plugin, you should try Really simple SSL. This plugin will transform all your non-secure URLs when it is enabled.