could anyone explain in detail \"List<List<sObject>>" in Salesforce?

List<List<sObject>> --> a list of a list of objects?

What does that mean and where is that used?
it means you have a list of a list of objects such as a list of account list, contact list, opportunity list

it is use main in SOSL, or also known as global search.  For example when doing a global search on "johnson"

You get a list of accounts, list of contacts, list of opportunities etc match the criteria

String searchQuery = 'FIND '' + searchStr1 + '' IN ALL FIELDS RETURNING  Account (Id,Name,type),Contact(name,email),Opportunity(name,StageName)';   List<List <sObject>> searchList = search.query(searchQuery);
accList = ((List<Account>)searchList[0]);
conList = ((List<contact>)searchList[1]);
optyList = ((List<Opportunity>)searchList[2]);

The example above has List<List<sObject>> of accounts, contacts, and opportunities using the FIND search in SOSL.