Purchase Sub Form 54

Hi

 I have written the below code . I want to know how these lines function

1. No. - OnLookup(VAR Text : Text) : Boolean

2. IF PAGE.RUNMODAL(0, ItemRec) = ACTION::LookupOK THEN

3. What is Rec,xRec in C/Al Symbol Menu

PHRec is Purchase Header Record SubType

No. - OnLookup(VAR Text : Text) : Boolean
PHRec.RESET;
PHRec.SETRANGE("No.", "Document No.");
IF PHRec.FINDFIRST THEN;

ItemRec.RESET;
ItemRec.SETRANGE("Vendor No.", PHRec."Buy-from Vendor No.");
IF PAGE.RUNMODAL(0, ItemRec) = ACTION::LookupOK THEN

Text := ItemRec."No.";

Thanks