Your title tag and meta description are the advert for your page in search results. Paste your page content or describe the page, and get title and description options written to the pixel limits Google actually truncates at, with the primary keyword placed naturally near the front.
Your result will appear here.
Around 150 to 160 characters. Google truncates by pixel width rather than character count, so front-load the important part.
Not directly, but it heavily affects click-through rate from the results page, and that traffic signal matters.
Not always. Google rewrites descriptions when it thinks another snippet better matches the query. A strong description still wins most of the time.