Its the level of isolation a transaction uses. Databases support different isolation levels that usually represent a trade of of data integrity vs. performance. For example the isolation level TRANSACTION_SERIALIZABLE means the database treats the transactions as if they are serial (i.e. one transaction at a time) so you get high guarentees of data integrity, but poor performance. At the other end you have TRANSACTION_READ_ UNCOMMITTED which allows other transations to read data that has yet to be committed. So you will get what are know as "dirty reads", but much improved performance since nothing is blocking a read.
Have a read of the JavaDocs for java.sql.Connection, it has some static variables defining the transaction isolation levels a database may support.