how the IsValid () function works?

Can this be use if the data I entered on a fields belongs to the value list.

See the help for the 3 scenarios where IsValid() will return a 'false' result.  The first two are the most common:

- check to see if the data in the field conforms to the data type of the field (date, number,...)

- check to see if the relationship to another table is valid

https://fmhelp.filemaker.com/help/18/fmp/en/#page/FMP_Help%2Fisvalid.html

You cannot use it to check if a value is part of a value list, for that you can use the field-level validations, or manually check as part of a trigger or navigation script by using the "ValueListItems()" function in combination with "FilterValues()" and the value from the field.