Your domain’s A record tells the world where your website lives. The computer your website lives on will have an IPV4 address assigned to it. This usually looks something like “126.96.36.199”. Chances are that the computer your website lives on is shared with other websites too. So multiple domain names can have the same A record.
Your A record is part of your domain’s DNS Zone (link).
If you ever change your hosting service you will need to change your A record manually. Most of the time this is done through your domain registrar (link). GoDaddy, Namecheap, and Network Solutions are examples of domain registrars.