Meta titles (page titles) and meta descriptions are two different pieces of the HTML code, which are important when you are creating a web page. Much like the back-cover blurb of a book, these elements will tell users the title of the page they have landed on and will give them the summary of that page when it shows up on SERPs. This gives the users an idea of what your page is all about and whether it will be relevant for them to visit your website.
Moreover, web browsers use the meta titles in the browser bars so that it is easy to distinguish between different tabs of different pages if the users have opened multiple tabs in a session. The page titles are also used when people bookmark a web page for future reference.