Describe the page, add the target keyword, and get three meta descriptions of 150 to 160 characters, each with the keyword in it and a character count.
Write one or two sentences about what the page covers and who it is for, then add the keyword you want it to rank for. Choose a tone and press Generate. You get three descriptions, each written from a different angle: one leads with the benefit, one raises a question, one tells the reader what to do next. Copy the one that fits, paste it into the meta description field in your CMS, and preview it with the SERP preview tool.
The meta description is the grey text under the blue link in a search result. It does not affect rankings directly, but it changes how many people click, and Google rewrites it when it does not match the query. A description that repeats the searcher's words and gives them a reason to click keeps Google from replacing it and lifts the click-through rate of a page that already ranks.
150 to 160 characters on desktop, and closer to 120 on mobile, where Google cuts it sooner. Put the important part in the first 120 characters so it survives on both.
Google swaps in text from the page when it thinks that text matches the search better. Including the target keyword and describing the page accurately makes your own version more likely to be used.
Not directly. Google has said the description is not a ranking factor. It does affect clicks, and a page that gets more clicks from the same position earns more traffic without moving up.