Backend means web development. It is the name given to the codes that are not seen by the users, that work in the background and that we can describe as the basis of the site. Programs such as PHP are written using their language, and the system is made operational by database systems such as mysql. For example, registering the e-mail address of the person who will register on your site to the database is an action with the back-end. Users cannot see or access these codes, they can only access front-end codes. The vast majority do not.
Having your site design ready has a positive impact on you in many areas, one of the examples is that fullstack developer does both design and web development, so it is more costly, but you can set up your site with a more affordable cost through a back-end developer for your ready-made site.
