explain plan parameter meanings

In the explain plan what is meant by "cost" , "card" and "byte" ? How may they be interpreted practically ? Any Docs , Links on the Same ?    

Execution Plan

   0 SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=1 Bytes=76)    1 0 NESTED LOOPS (Cost=6 Card=1 Bytes=76)

   2    1     NESTED LOOPS (Cost=5 Card=1 Bytes=50)
   3    2       TABLE ACCESS (BY INDEX ROWID) OF 'GENERAL_ACCT_MAST_TA
          BLE' (Cost=3 Card=1 Bytes=17)
 
   4    3         INDEX (UNIQUE SCAN) OF 'IDX_GAM_FORACID' (UNIQUE)

   5 2 TABLE ACCESS (BY INDEX ROWID) OF 'ASHT_DTD' (Cost=2 Card=5830 Bytes=192390)  

   6 5 INDEX (RANGE SCAN) OF 'IDX_ASHT_DTD_ACID' (NON-UNIQUE)
(Cost=1 Card=5830)
 

   7 1 TABLE ACCESS (BY INDEX ROWID) OF 'ASHT_TDTD' (Cost=1 Card=2105 Bytes=54730)  

   8 7 INDEX (UNIQUE SCAN) OF 'IDX_ASHT_TDTD' (UNIQUE)   Thanks  

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