Check Oracle Error with OERR utility
OERR utility is used to check about the error information by code provided by oracle during execution of Oracle processes.
oerr prefix number
Prefix means EXP, ORA, PLS, IMP etc
Number means error number 02095,01555 etc
OERR utility example with snapshot old.
C:\Users\oracle>oerr ora 01555
01555, 00000, "snapshot too old: rollback segment number %s with name \"%s\" too small"
// *Cause: rollback records needed by a reader for consistent read are
// overwritten by other writers
// *Action: If in Automatic Undo Management mode, increase undo_retention
// setting. Otherwise, use larger rollback segments
C:\Users\oracle>oerr ora 00020
00020, 00000, "maximum number of processes (%s) exceeded"
// *Cause: All process state objects are in use.
// *Action: Increase the value of the PROCESSES initialization parameter.