The validation type determines how the form validation will work for this question. If mandatory is set to no, then this setting is ignored so it doesn’t matter what is set. When mandatory is set to yes, the validation type can be set, and the options available will be different depending on the question type. For example, text fields have required validate-numeric, whereas checkbox will have required validate-checkbox.
For text fields, the default of required will just ensure that the field is not left blank, whereas required validate-email will force the user to enter an email address (please note that it does not check that the email address is valid, it just checks the format, so potentially someone could still enter a fake email address such as email@example.com).
For checkbox question type, you can choose the exact number of options you want selected, for example if you want the user to select exactly three options, you could set required validate-checkbox3 (thiis requires BF Validate Plus plugin is installed and published).
For some question types there is only one validation type available, so it will just say default.