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Your Domain Name is going to be the thing that you will need people to remember, as it will be the address to get to your website.
Your website is made up of usually multiple files, making up the style, layout and content of your website. These files are stored on a hosting server connected to the internet. The server that stores your website files has a unique address, much like you can be found at your postal address, but these server addresses are not very user-friendly.
To resolve this problem, a Domain Name is used instead, which links directly to your ugly computer address and directs visitors who visit your Domain Name to your website files, and therefore displays your website.
A Domain Name is therefore extremely important, as the whole point of it is to make the address to your website memorable to visitors.