This means the file or page that the browser is requesting wasn’t found by the server. 404s don’t indicate whether the missing page or resource is missing permanently or only temporarily. You can see what this looks like on your site by typing in a URL that doesn’t exist. It’s like hitting a brick wall. Just as you’ve experienced, your visitors will hit a page that has a 404 error and either try again (if you’re lucky) or wander away to another site that has the information they’re seeking.
Every site will have some pages that return 404 status codes. These pages don’t always have to be redirected; there are other options. One common misconception is that it’s an SEO best practice to simply 301 redirect pages that return a 404 status code to the homepage of the given domain. This is actually a bad idea for the majority of cases, because it can confuse users who may not realize that the webpage they were trying to access doesn’t exist.
If the pages returning 404 codes are high-authority pages with lots of traffic or have an obvious URL that visitors or links are intended to reach, you should employ 301 redirects to the most relevant page possible. For example, if your page on sugar-free cupcakes no longer exists, you may want to redirect this URL with a 301 to your sugar-free recipe category page.