JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Responsive form is a form that will modify itself in terms of layout so that it is always best represented to your end users.
That way if your form is opened on desktop, it will open as you have designed it, but when it is opened on a mobile phone, it will change its layout so that fields are large enough for anyone with the mobile device to fill it out without any issues.
You can test this in your Form Designer through Preview option.
You can see more about this here: Episode 8: Device Previews
Part of the video course: Video Course: Form Designer Basics