MyBB Community Forums

how can i edit my forum description so it can be shown when people search for my website on google?

Like you website description? You can add this code in your headerinclude template.

<meta name="description" content="Your website description here." />

 
also you can add this under it if you want

<meta name="keywords" content="Your wesbite, forum, example" />
<meta name="author" content="Your Name" />

To access your headerinclude go to
Admin CP -> Templates & Styles -> Templates -> * Your Theme * -> Ungrouped Templates -> headerinclude

or you can try this plugin Add Meta Tags.

when you llook to my forum on google it show no description for it how can i edit that?

Please do exactly what was mentioned above. Adding a meta description will show it on google once the Google crawler reads your site again.... This may take a while!