A 404 error shows when a page is not found.
Sometimes this can be because a customer clicks on a link which leads nowhere, or maybe they have typed a URL manually, which then takes them to a non-existent part of your site. You may have sent out a link to a product or page on social media and then at a later date, removed or altered the location of this.
When the customer hits that endpoint, where the location is no longer valid, they will see a 404 error.
You have a 404 error page and can edit the text the customer sees.