[Vivek]

In simple terms, You login to your Facebook app or Salesforce one app once using your credentials . But it keeps you logged in forever that is because of refresh token. Access token expires after sometime based on your session settings. But using refresh token app gets new access token whenever it expires.

[Vivek]

It is very simple assume you have a Set<String> or Set<Id> named "ids"

Set ids = new Set();

String dynamicQuery = "SELECT ID FROM SomeObject__c WHERE SomeID IN :ids";

Database.query(dynamicQuery);

You don't have to do anything when complier will compile the code it will find set of ids and put it into your query