Actually a tool-tip like on desktop but used for mobile info. For example, on my form, I am decorating fields with an icon if the user might not understand the intent of a field. I also decorate them with the icon if they have failed validation.
Right now, if the user taps the icon, I’m using the native notification to display whatever info I want them to see. It works, but I’m slightly worried apple may frown on it. I designed my own CSS based callout, but it looked too ugly; so, I reverted back to the native notification.
Examples : Note the icon in the ‘Secret Pin’ field:
I didn’t show it here, but I also change the field separator to red on error. These techniques seem the best way to deal with validation on mobile for me. Otherwise, the keyboard tends to hide all the validation info.