Hi,
You dont have to understand alert log file on daily basis.Basically alert log file contains important error messages such as ORA-00600 ,deadlock error messages.Apart from this any changes made to the database by ALTER DATABASE or ALTER TABLESPACE command is recoreded in alert.log file.Also log switches and checkpoint intervals are recoreded in alert.log file.YOu can monitor alert.log file to see any ORA-00600 errors(bugs) are found in alert.log file and also to tune your checkpoint intervals and redo log switches you can see it.

In case of any help please be free to ask me at [email protected]

Regards.
Rohit Nirkhe,Oracle DBA,OCP 8i
[email protected]