The page description field in Weebly fills in the meta description on your web page. If the page description is manually set, then Google will use this as the Google SERP Snippet. When the page description on a Weebly website is not set, Google takes the first 160 characters (no matter WHAT they are) on your website to build the snippet.
The Google SERP Snippet below is an example of a snippet generated by Google on the homepage of a website that didn't write its own meta description. This example is typical on eCommerce websites that don't write their own meta descriptions. In this case, you can see it pulled the content from the top of the eCommerce page. This doesn't describe the actual web page .