Meta Descriptions on Sites?

Hi! I'm kind of new to all of this. I use Monsido to track for accessibility (side note: it keeps flagging a lot of the sites template as being inaccessible for things like color contrast and links being out of context). Monsido suggested that I could add meta descriptions to help improve SEO. I googled some code, but whenever I add it, it disappears from html section as soon as I switch back to visual.