please tell me what is there in ST22, why we use it.
You can use the tools of the ABAP dump analysis (ST22) to list the ABAP runtime errors that have occurred in an ABAP system as well as the relevant short dumps.
Displays and analyzes update statistics again
Save in a local file
Store for a longer period of time
ST22 ABAP Runtime Error Analysis
ST22 Check and analyze SAP ABAP short Dumps An ABAP dump (AKA short dump) is generated when a report or transaction terminates as the result of a serious error. The system records the error in the system log. SM21 and write a snapshot (dump) of the program terminiation to a special table (SNAP). This transaction can also be called from the system log (SM21). ABAP dump is used to analyze and determine why the error occured and take corrective action
SM21 Check and analyze SAP system log for any critical log entries (hourly)