When you have purchased your hosting, you will have to connect your domain with the hosting that you have purchased. If you have purchased the domain and hosting from the same provider, then it is liable that your Nameservers are already setup. If you have purchased your domain and hosting from different providers, then you will have to manually add new Nameservers to your domain, so that it can point your domain to the right server.
Nameservers are a part of Domain Name System. These DNS servers keep the records of which domain is connected to which IP address. Nameservers allow us to use a domain instead of IP address of the DNS server. When a visitor types the URL of your domain then the DNS server tells the browser that from which server it should load content from. In easy words, Nameservers define the current DNS provider of your domain.
When you get the email of the Login details of your web hosting server along with it, you also get the details of your Nameservers. Most hosting providers use their own DNS servers. For setting up your Nameservers, you will have to login to your domain control panel, and update your Nameservers by replacing the old one. Each domain provider has its own application to do so, you will have to refer to your domain registrar’s documentation for further details.
Domain Nameservers take time to propagate through different DNS servers around the globe, the change may reflect instantly or it may take up to 48 hours of time according to the DNS servers of hosting provider.