This attribute is useful for putting input fields outside the form itself. The form attribute of an external input field must share the same value as the id of the form the field belongs to. This is useful when using <fieldset> elements for making the page/form layout easier.
Try this interactive example in CodePen.
Source code:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Example of input type=tel</title>
</head>
<body>
<label for="yourName">Enter your name:</label>
<input type="text" id="yourName" name="yourName" form="form1"/>
<p>
<form id="form1" action="sumit.php" method="post">
<fieldset>
<legend>Choose option</legend>
<label for="free">Free registering</label>
<input type="checkbox" id="free"/>
<label for="premium">Premium</label>
<input type="checkbox" id="premium"/>
<button type="submit">Send form</button>
</fieldset>
</form>
</body>
</html>
Lines 12 and 22 shows the form attribute. Make sure that its value matches the id of the form!