Salesforce Validation Rules Part-2: ISBLANK Function 

The ISBLANK function in Salesforce checks if a specified field is empty. It returns TRUE if the field is empty and FALSE if it contains any data.
Key Points About ISBLANK:
Syntax:
ISBLANK(field)
Here, field is the one you want to check.
Usage:
Used in Validation Rules, Workflow Rules, and other formulas to check if a field is filled out.
Helps ensure required fields are not left empty or triggers logic when a field is blank.
Examples:
To ensure the "Phone" field isn't left blank: ISBLANK(Phone)
To check if both "Phone" and "Email" fields are blank: AND(ISBLANK(Phone), ISBLANK(Email))
Consideration:
If a user enters a space, ISBLANK may return FALSE since the field isn't technically empty. Use the TRIM function with ISBLANK to check for fields with only whitespace.
Overall, ISBLANK is a versatile function for managing data entry and ensuring required fields are completed.