what is meant by non-atomic value in database because in website no information about this....

An atomic value in logical mathematics is something like:
variable
!variable

A non-atomic value (if I remember correctly) is:
variable mod variable2

Now, how that relates to your database question I'm not too sure but it might help you figure it out. Look up some logical maths.

There's no such thing as an atomic value in a database, but there's such a thing as an atomic update. This is basically a transaction, in that it either happens, or it doesn't (ie happens and is removed, with no lasting effects).