In salesforce SOQL there are two type of subquery.

1. from parent object to the child object subquery .

suppose you try to fetch Account(parent) records and related all Contacts(child)

then use this subquery

list<Account> lstAccountToContact = [select Id,Name,(select Id,Name from Contacts) from Account];

system.debug(lstAccountToContact.get(0).Contacts.Name);

2. from child object to the parent object subquery.

list<Contact> lstContactToAccount = [select Id,Name,Account.Id,Account.Name from Contact];

system.debug(lstContactToAccount.get(0). Account.Name);