What is the use of gethost() method in Salesforce apex?

Hi Tannu,
The URL instance method getHost() is used to get the hostname of the Salesforce server that you are using. For instance, if your Salesforce instance is ap1 then you would get the result as "ap1.salesforce.com".

In case of custom domains, suppose your org has a custom domain "my-dev-org" the method would return "my-dev-org.ap1.my.salesforce.com"

You can try out the function in anonymous execution block:

system.debug(URL.getSalesforceBaseUrl().getHost());