If the people visiting your website are filling out a form incorrectly, you could add some instructions to the form to make it clearer.
Or you could just redesign it to remove the confusing part.
When faced with a problem, it's easy to focus on additive solutions ("How do I instruct people to do this correctly?"), rather than subtractive ones ("What can I remove so that this needs no instruction?")
One study showed that when tasked with making a pattern of blocks symmetrical by either adding or removing blocks, only 20% of people chose to remove them.
Our minds are naturally drawn to additive solutions. But often, you can make a problem go away by simply removing the source of it, rather than adding a solution.
See also: Simplicity is Hard