How do I add my title and description of my website in the meta tags?

I recommend doing it in the headerinclude template

templates > ungrouped templates > headerinclude

@BadgeR There would be a specific plugin for this in the official MyBB.com mods list! And I also think that it's not important to add the meta tags in the head section of your website, whatever CMS it is anymore! Search Engines do not look at them as they did before anymore!