REPEATABLE_READ is an isolation level higher than READ_COMMITTED
A trans T2 may have modified data read by T1 , yet when T1 reads the data again it does not see the modified value by T2.ie T1 is isolated from the fact that T2 has modified the data it has read.

In other words in this isolation level, T1 can read some data, while T2 can read the same data and modify it yet T1 will not consider those changes.

  session1> BEGIN;
  session1> SELECT firstname FROM names WHERE id = 7;
  Amit

  session2> BEGIN;
  session2> SELECT firstname FROM names WHERE id = 7;
  Amit
  session2> UPDATE names SET firstname = 'Ramesh' WHERE id = 7;
  session2> SELECT firstname FROM names WHERE id = 7;
  Ramesh
  session2> COMMIT;

  session1> SELECT firstname FROM names WHERE id = 7;
  Amit

Note that session 2 has actually modified the data, yet session 1 still reads unmodified data
this can lead to missing update problem.