Tuning Question

select /*+ USE_MERGE(B,A) */ a.prod_id,
       a.prod_number,
       a.prod_name,
       a.update_date,
       b.term,
       b.term_type
  from merch a,
       term_details b
 where a.prod_id = b.prod_id

Experts,
  I just used TOAD to tune the following query and received this result as the best option. Before I use the code I wanted to understand what the solution was. I was hoping someone could explain the /* + USE_MERGE(B,A) */ comment? I don't want to use code and not fully understand what it's doing. Thanks!