Please explain this stored procedure and how it works

Create PROCEDURE  [dbo].[usp_zcCandidate_SELECT_By_Client_Status_PayRollType]       

(      

 @ClientID int = NULL,      

 @StatusID int = NULL,      

 @PayrollTypeID int = NULL ,  

 @LoggedInUserID UNIQUEIDENTIFIER      

)        

AS      

    

if (@ClientID = 0)    

set @ClientID = null    

    

if (isnull(@StatusID, -1) =-1 )    

BEGIN     

  SELECT  CandidateID, FirstName +' '+ LastName as CandidateName, DateCreate,         

    C.ClientID as ClientID, ClientName, C.StatusID, S.[Name] as Status, IsRehire        

  FROM zcCandidate C        

  LEFT JOIN zcCandidateStatus S ON C.StatusID = S.StatusID  

  INNER JOIN fn_GetAccessibleClientIDsByLoggedInUserID(@LoggedInUserID) AS AC ON C.ClientID = AC.ClientID    

  WHERE     

   (C.ClientID = ISNULL(@ClientID, C.ClientID))        

    AND ((C.StatusID = 1 OR C.StatusID = 8))    

    AND  IsPayrollee1099 & @PayrollTypeID > 0      

  ORDER BY DateCreate, LastName      

END     

 ELSE    

BEGIN    

 SELECT  CandidateID, FirstName +' '+ LastName as CandidateName, DateCreate,         

    C.ClientID as ClientID, ClientName, C.StatusID, S.[Name] as Status, IsRehire        

  FROM zcCandidate C        

  LEFT JOIN zcCandidateStatus S ON C.StatusID = S.StatusID  

  INNER JOIN fn_GetAccessibleClientIDsByLoggedInUserID(@LoggedInUserID) AS AC ON C.ClientID = AC.ClientID    

  WHERE     

   (C.ClientID = ISNULL(@ClientID, C.ClientID))        

    AND C.StatusID = @StatusID     

    AND  IsPayrollee1099 & @PayrollTypeID > 0      

  ORDER BY DateCreate, LastName     

END