Client-side rendering are any changes made to a website after it's loaded in the visitor's browser. The opposite is server-side rendering, where a complete HTML source is send to the browser.
Client-side rendering has become popular with the rise of front-end frameworks like Vue, Angular and React. Client-side rendering can harm SEO, because Google and other search engines only consider the server-side variant at first. Any content that is added later on, may not be seen by the search engine, or only be seen., crawled and indexed later on. As a result, a website that uses a lot of client-side rendering can harm SEO activities.
Most of the popular front-end frameworks now offer a way to add server-side rendering.Back