Allows you to display a form field based on a condition which allows you to do something like "only display the field when value of field 'foo' is 'bar' and 'bla' is 'yes'".
$foo == 'bar' && $bla == 'yes'
You can also check the array (e.g. multiple checkboxes or select menu):
To validate a single checkbox simply compare its value:
$foo == '1'
Make shure jQuery is loaded for this to work.